Processes in which hydrocarbons are hydrogenated or dehydrogenated are effected in the presence of phosphorus pentoxide and in the absence of any other solid catalyst. As an activator for the catalyst, hydrogen sulphide, hydrogen chloride, hydrogen bromide, hydrogen iodide, hydrogen fluoride or hydrogen cyanide may also be present, generally in a proportion not exceeding one per cent by weight of the hydrocarbon; an activator may also be formed in situ from an alkyl chloride or nitrile. The invention may be applied to the simple hydrogenation or dehydrogenation of hydrocarbons, e.g. the conversion of aromatic into hydroaromatic hydrocarbons, or to the conversion of olefines into branched-chain paraffin hydrocarbons by combined polymerization and hydrogenation, or to the refining of gasoline by hydrogenation, all these processes being effected at a raised temperature and pressure. In the examples, (1) benzene is converted into cyclohexane by treatment with hydrogen at 300 DEG C. and 100 atm. in the presence of phosphorus pentoxide and subordinate proportions of lamp-black and cresol; (2) butene-1 is treated with hydrogen at 250 DEG C. and 100 atm. in the presence of phosphorus pentoxide and subordinate proportions of lamp-black and cresol; a mixture of saturated hydrocarbons including isoparaffins is obtained; (3) diisobutylene is treated with hydrogen at 400 DEG C. and 50 atm. in the presence of phosphorus pentoxide and lamp-black; a gaseous product consisting mainly of butane, and a liquid product containing unsaturated and aromatic hydrocarbons, are obtained; (4) the process of example 3 is effected at 387 DEG C. and 68 atm., similar products being obtained; (5) butene-1 is treated with hydrogen at 200 DEG C. and 100 atm. in the presence of phosphorus pentoxide which has been pretreated with hydrogen sulphide, hydrogen chloride, or hydrogen cyanide; small proportions of lamp-black and cresol are also present; a product resembling that of example 2 is obtained; (6) cyclohexane vapour is converted into benzene by passage at 400 DEG C. over a mixture of phosphorus pentoxide and lamp-black; higher yields are obtained if hydrogen chloride or hydrogen sulphide is added; (7) a vapour-phase distillate boiling at 80-250 DEG C. is treated with hydrogen at 400 DEG C. and 100 atm. in the presence of phosphorus pentoxide, lamp-black and cresol, and with the optional addition of hydrogen chloride; gasoline of octane number 82 (or 86) is obtained; the cresol added in some of the examples acts as a peptizing agent for the phosphorus pentoxide, and the lamp-black (except in example 6) serves as a dispersion-stabilizing agent, as described in Specification 437,072; the lamp-black used in example 6 acts as a distributing agent for the phosphorus pentoxide. The conversion of ethylene into a mixture of saturated hydrocarbons by treatment with hydrogen at 250 DEG C. and 100 atm. in the presence of phosphorus pentoxide is mentioned. Specification 471,482 also is referred to. The Provisional Specification mentions water as an activating agent for the phosphorus pentoxide, and refers to mercaptans as reagents for generating hydrogen sulphide as an activator in situ.ALSO:Processes in which hydrocarbons are hydrogenated or dehydrogenated are effected in the presence of phosphorus pentoxide and in the absence of any other solid catalyst. As an activator for the catalyst, hydrogen sulphide, hydrogen chloride, hydrogen bromide, hydrogen iodide, hydrogen fluoride or hydrogen cyanide may also be present, generally in a proportion not exceeding one per cent by weight of the hydrocarbon; an activator may also be formed in situ from an alkyl chloride or nitrile. The invention may be applied to the simple hydrogenation or dehydrogenation of hydrocarbons, e.g. the conversion of aromatic into hydroaromatic hydrocarbons, or to the conversion of olefines into branched-chain paraffin hydrocarbons by combined polymerization and hydrogenation, all these processes being effected at a raised temperature and pressure. In the examples: (1) benzene is converted into cyclohexane by treatment with hydrogen at 300 DEG C. and 100 atm. in the presence of phosphorus pentoxide and subordinate proportions of lamp-black and cresol; (2) butene-1 is treated with hydrogen at 250 DEG C. and 100 atm. in the presence of phosphorus pentoxide and subordinate proportions of lamp-black and cresol; a mixture of saturated hydrocarbons including isoparaffins is obtained; (3) diisobutylene is treated with hydrogen at 400 DEG C. and 50 atm. in the presence of phosphorus pentoxide and lamp-black; a gaseous product consisting mainly of butane, and a liquid product containing unsaturated and aromatic hydrocarbons, are obtained; (4) the process of example 3 is effected at 387 DEG C. and 68 atm., similar products being obtained; (5) butene-1 is treated with hydrogen at 200 DEG C. and 100 atm. in the presence of phosphorus pentoxide which has been pretreated with hydrogen sulphide, hydrogen chloride, or hydrogen cyanide; small proportions of lamp-black and cresol are also present; a product resembling that of example 2 is obtained; (6) cyclohexane vapour is converted into benzene by passage at 400 DEG C. over a mixture of phosphorus pentoxide and lamp-black; higher yields are obtained if hydrogen chloride or hydrogen sulphide is added; the cresol added in some of the examples acts as a peptizing agent for the phosphorus pentoxide, and the lamp-black (except in example 6) serves as a dispersion-stabilizing agent, as described in Specification 437,072; the lamp-black used in example 6 acts as a distributing agent for the phosphorus pentoxide. The conversion of ethylene into a mixture of saturated hydrocarbons by treatment with hydrogen at 250 DEG C. and 100 atm. in the presence of phosphorus pentoxide is mentioned. Specification 471,482 also is referred to. The Provisional Specification mentions water as an activating agent for the phosphorus pentoxide, and refers to mercaptans as reagents for generating hydrogen sulphide as an activator in situ.