In a hydrocarbon conversion process wherein hydrocarbon fractions are cracked over acid catalysts at elevated temperatures, the improvement comprising using as the catalyst a material comprising a catalytic metal oxide selected from the group consisting of the oxides of tungsten, niobium, and mixtures thereof and tungsten or niobium oxides in combination with one or more additional metal oxides selected from the group consisting of tantalum oxide, hafnium oxide, chromium oxide, titanium oxide and zirconium oxide supported on an inorganic refractory metal oxide base selected from the group consisting of alumina, zirconia, boria, thoria, magnesia, zirconium-titanate, titania, chromia, kieselguhr and mixtures thereof, said catalyst being steamed prior to use.