Abstract: Hydrocarbon catalytic reactions (including isomerization, cyclization, aromatization, disproportionation, alkylation, de-alkylation, reforming, hydrcracking) are effected over a catalyst comprising a crystalline solid solution of two or more irreducible compounds differing in the valency of their cations and/or their anions. The catalysts may be made by heating together the components of the crystalline solid solution or compounds thermally decomposable thereto. Some of the catalysts contain one or more additional acidic materials which act as co-catalysts, and are new compositions of matter.