The conversion of nickel and vanadium porphyrins under conditions of catalytic cracking has been studied. Samples of porphyrins and their mechanical mixtures with a cracking catalyst have been investigated by thermogravimetric analysis. As a result of the cracking of porphyrins, a significant amount of gaseous and liquid products are formed. The liquid products are mainly represented by polycyclic compounds. A possible mechanism of nickel and vanadium porphyrin conversions has been suggested.