Abstract: A process for preparing an activated isomerization or alkylation catalyst composition in which a catalyst composition comprising a Group VIII noble metal and a hydrocarbon-substituted aluminum compound on an alumina carrier containing up to 20 wt. % of other components involves activating the catalyst by contacting the catalyst with a hydrogen-containing gas at a temperature in excess of 500.degree. C., with the proviso that at least when the hydrocarbon-substituted aluminum compound present in the catalyst composition is not a hydrocarbon-substituted aluminum halide, the catalyst composition to be activated is contacted with a halogen compound either prior to or during the activating step. The catalyst obtainable by this process and the use thereof in hydrocarbon conversion processes, such as isomerization and alkylation processes, are also part of the invention.