A process for converting heavy hydrocarbon oils continuously to low-boiling oils is carried out in apparatus comprising a vaporizing zone 7 and two heating and conversion systems, so that the oils or products may be subjected to different conditions of temperature, pressure and reaction time in the two systems in order to obtain maximum yield without undesirable products. Each heating and conversion system consists of a heating zone, preferably of the nature of a coil 23 or 123, and a conversion zone preferably of the nature of one or more conversion chambers 30, 32 or 140, 141. The system 23, 30, 32 in which the cracking conditions are less severe is called the primary heating and conversion zone, and the system 123, 140, 141 is called the secondary heating and conversion zone. The conditions in the primary zone may be, for example, 800-1000 DEG F. and 100 lbs. per sq. in. pressure in the coils, and 750-850 DEG F. in the conversion chambers, and in the secondary zone 900-1500 DEG F. and 200 lbs. per sq. in. or more in the coils and 800-900 DEG F. in the conversion chambers, but higher temperatures and pressures are sometimes necessary in both zones, and the pressures may be as high as 3000-5000 lbs. per sq. in. The pressures in the conversion chambers in each zone correspond with those in the coils but are lower. The pressures in the secondary zone are always higher than in the primary zone. The pressure in the vaporizing zone 7 may be the same as in the primary heating and conversion zone, or lower, or atmospheric, and the temperature is regulated so that the vapours withdrawn at 55 consist mainly of gasoline, naphtha and light kerosene, the heavier fractions forming an intermediate condensate in <PICT:0359027/III/1> the pan 18 and a liquid residue below. The temperature of this residue is preferably maintained at 600-700 DEG F. The process is carried out in several different forms: (1) The feed oil from the source 1, Fig. 1, preheated if desired in coils 8, 9, 4, 16, passes through lines 28, 28<1> and is cracked or hydrogenated in the primary heating and conversion zone 23, 30, 32; the products are discharged into the vaporizer 7; an intermediate condensate from the vaporizer passes through lines 19, 22 and is cracked or hydrogenated in the secondary heating and conversion zone 123, 140, 141; the products are discharged into the vaporizer through line 145; the vapours are withdrawn at 55 and the residue from the vaporizer is withdrawn at 71 and may be submitted directly to destructive hydrogenation. (2) The feed oil passes through lines 28, 155, Fig. 1, and is cracked or hydrogenated in the secondary zone 123, 140, 141; the products are discharged into the vaporizer 7; the residue from the vaporizer passes through line 45 and is cracked or hydrogenated in the primary zone 23, 30, 32; the products are discharged into the vaporizer through line 98, the vapours are withdrawn at 55, and part of the residue may if necessary be withdrawn; and in a modification, an intermediate condensate from the vaporizer is passed through lines 19, 22, and branches such as 39 to the inlet or to an intermediate point or points of the secondary zone. (3) The feed oil from the source 1, Fig. 2, preheated <PICT:0359027/III/2> if desired in coils 8, 9, 4, passes through line 6<1> into the vaporizer 7; an intermediate condensate passes through lines 19, 22 and is cracked or hydrogenated in the secondary zone 123, 140, 141; the products are discharged into the vaporizer through line 145; the residue from the vaporizer passes through line 45 and is cracked or hydrogenated in the primary zone 23, 30, 32; the products are discharged into the vaporizer through line 98, and the vapours are withdrawn at 55. Part of the residue may if necessary be withdrawn. In forms (1), (2) and (3) of the process the products discharged from the primary and secondary zones are preferably intimately mixed together and enter the vaporizer through a common line 35<1>. (4) The feed oil, which may be preheated as above, passes through line 6<11>, Fig. 3, into a still 160 at atmospheric pressure or above, and is preferably spread over a tilted tray 161; the residue passes through lines 164, 45 and is cracked or hydrogenated in the primary zone 23, 30, 32; the products are discharged into the vaporizer through line 98; an intermediate condensate from the vaporizer passes through lines 19, 22, and is cracked or hydrogenated in the secondary zone 123, 140, 141; the products are discharged into the vaporizer through line 145; and the vapours are withdrawn at 55. The residue is withdrawn from the vaporizer at 71 and may be subjected directly to destructive hydrogenation. The products from the primary and secondary zones may be intimately mixed together and preferably also with the vapours from the still 160, and enter the vaporizer through a common line 35<1>. In all forms of the process where applicable, and as shown with references in Fig. 1, (a) the hydrogenating materials may be as set out in Specification 359,026, with or without catalysts, and they may be introduced at the inlet and at other points of the heating and conversion zones through lines 104, 117 and branches 105, 118, 119, 120, 107, 108, 121, 122; the hydrogenating gases are drawn from a source 99 and preheated in furnace 102; (b) hydrogenation may be promoted by the use of catalysts, including molybdenum and tungsten catalysts, in any of the coils and conversion chambers; the catalysts may be spread on trays in the chambers, or circulated with the oil, in suspension or in colloidal solution; and catalysts or hydrogenating materials may be placed in the coil sections 27, 127, which may be of larger diameter; (c) gases and vapours not condensed in working up the light products may be purified and returned to the hydrogenation system, and these gases and vapours, or additional gases such as natural gas or refinery gas, or a mixture of such gases, may be subjected to dissociation temperatures as described in Specification 359,026, before use as hydrogenating gases; (d) the materials circulating in the secondary heating zone 123 may be led through tubes 132, 133 in the primary conversion chambers 30, 32 before they pass to the secondary conversion chambers, in order to regulate and assist to maintain the required temperature; (e) the conversion chambers 30, 32 may be worked in series or in parallel, as also may the chambers 140, 141; they may be fitted with <PICT:0359027/III/3> agitators, or the materials undergoing conversion may be recirculated through them, or through the coils 23, 123; (f) intermediate condensate from the vaporizer, or cool oil from other parts of the system, may be introduced through a branch 19<1>, Figs. 1 and 2, into the stream of products entering the vaporizer in order to control the temperature; (g) the vapours withdrawn from the vaporizer at 55 are worked up in fractionators 11, 10, cooled by preheating coils 8, 9 for the feed oil, and by independent coils 58, 59, and the condensate may be returned to the vaporizer through the line 79, or passed through coolers to storage; (h) intermediate condensate from the vaporizer 7, which is to be passed to the secondary zone, may first be treated in the still 66 to separate the lighter fractions; (j) the vapours from the still 66 may be fractionated separately, or passed to the fractionators 11, 10 or the vaporizer 7 or to both; (k) residue from the vaporizer 7 which is not to be passed to the primary zone may be treated in an auxiliary still or stills 74, 75; the vapours from the stills 74, 75 may be treated like those from the still 66, or they may first be fractionally condensed at 85 and the condensate returned through line 89 to the heating and conversion system or through line 88 to the still 66; and the treatment of the residue in the stills 74, 75 may be carried to dryness, or a final liquid residue may be withdrawn, or returned through line 113 to the heating and conversion system; (1) when the feed oil is passed into the vaporizer before conversion, the vaporizer is constricted as described in Specification 359,022, so that the heavier unvaporized fractions do not mix with the intermediate condensate collecting in the pan 18; (m) live steam may be introduced into the vaporizer 7 at 15, into the fractionators 10, 11, 85 at 60, 85<11>, into the stills 66, 74, 75 at 67, 76, and into the still 160, Fig. 3, at 162.