A process for converting feedstock hydrocarbon compounds to product hydrocarbon compounds having a lower molecular weight than the feedstock hydrocarbon compounds comprises contacting said feedstock at conversion conditions with a catalyst composition comprising a large-pore molecular sieve material and an additive catalyst composition comprising crystalline material having the structure of ZSM-5 and a silica/alumina mole ratio of less than 30, said crystalline material having, as-synthesized, the following formula on an anhydrous basis: (x)M2O: (0.2 to 1.4)R2O:Al2O3: (y)SiO2 wherein x is a number greater than 0.1, M is alkali or alkaline earth metal, R is n-propylamine, and y is a number less than 30, and said additive catalyst composition having an alpha value greater than 30.