<PICT:0584746/III/1> In the catalytic conversion of hydrocarbons by passage with a hydrogen halide through a catalyst-containing conversion zone, the products being passed with the hydrogen halide and fixed gases to a product accumulating zone, and then into a fractionating zone wherein they are separated into a normally gaseous fraction comprising the hydrogen halide and a liquid hydrocarbon fraction, the fixed gases discharged from the accumulating zone are contacted with a liquid hydrocarbon to selectively dissolve the hydrogen halide, and all or a part of the resulting solution is passed to the fractionating or conversion zone. As described for butane isomerization, butane is fed through driers 12 and line 14 to isomerizer 15, hydrogen chloride equal to about 0.3-10 per cent. being introduced by line 19. The products are led by line 20 via coolers 22, 23 to accumulator 21, a liquid hydrocarbon, such as a butane fraction separated from the products, being introduced through line 24 in sufficient amount to cause solution of all the hydrogen chloride present. Solution from accumulator 21 is stripped in column 27, which may operate at above 300 p.s.i., and hydrogen chloride returned by line 19 to the isomerizer, any excess over that required as promoter being passed through valve 31 to line 20 for solution in the butane stream. Liquid from stripper 27 is passed to fractionator 37 except for the controlled amount feed to line 24. Isobutane is recovered as overhead while at least part of the n-butane is fed back by line 14 to the isomerizer. Gas from accumulator 21 is forced through line 51 to absorber 52 fed by line 53 with cooled butane from line 24. Fixed gases free of hydrogen halide are eliminated while the butane with absorbed hydrogen halide is returned by line 56 to line 24. Part may be combined with reaction products flowing to stripper 27. In a modification, part of the initial butane charge is fed to absorber 52 and the solution obtained led to line 14 and isomerizer 15ALSO:<PICT:0584746/IV/1> In the catalytic conversion such as isomerization, cracking, polymerization and alkylation of hydrocarbons or reaction of normally gaseous materials with a main hydrocarbon feed by passing with a hydrogen halide through a catalyst-containing conversion zone, the products being passed with the hydrogen halide and fixed gases to a product accumulating zone, and then into a fractionating zone wherein they are separated into a normally gaseous fraction comprising the hydrogen halide and a liquid hydrocarbon fraction, and fixed gases comprising the hydrogen p halide are discharged from the accumulating zone, the fixed gases are contacted with a liquid hydrocarbon to selectively dissolve the hydrogen halide, and all or a part of the resulting solution is passed to the fractionating or conversion zone. Butane may be fed through driers 12 and line 14 to isomerizer 15, hydrogen chloride equal to about 0.3-10 per cent being introduced by line 19. Vapour phase isomerization may be effected with, for example, aluminium chloride on alumina at 50-200 DEG C. The products are led by line 20 via coolers 22, 23 to accumulator 21, a liquid hydrocarbon, such as a butane fraction separated from the products, being introduced through line 24 in sufficient amount to cause solution of all the hydrogen chloride present. Solution from accumulator 21 is stripped in column 27, which may operate at above 300 p.s.i., and hydrogen chloride returned by line 19 to the isomerizer, any excess over that required as promoter being passed through valve 31 to line 20 for solution in the butane stream. Liquid from stripper 27 is passed to fractionator 37 except for the controlled amount fed to line 24. Isobutane is recovered as overhead while at least part of the n-butane is fed back by line 14 to the isomerizer. Gas from accumulator 21 is forced through line 51 to absorber 52 fed by line 53 with cooled butane from line 24. Fixed gases free of hydrogen halide are eliminated while the butane with absorbed hydrogen halide is returned by line 56 to line 24. Part may be combined with reaction products flowing to stripper 27. In a modification, part of the initial butane charge is fed to absorber 52 and the solution obtained led to line 14 and isomerizer 15. The liquid or vapour phase isomerization of other paraffins or naphthenes may be similarly effected. Specification 578,085 is referred to.