According to the energy gradient selectivity (EGS) concept, catalytic cracking is not fundamentally related to acidity, but primarily to the presence of intense electric fields due to polarised charge distributions that cause reacting molecules to crack. It was proposed that the ratio of C3/C4 in n-alkane cracking on zeolites may be considered as a field gradient selectivity index and is a measure for the ratio of the non-classical cracking mechanism via penta-coordinated carbonium ions and the classical beta-scission mechanism. This paper describes how the EGS concept can be used to characterise dealuminated zeolites in commercial FCC catalysts by gas-oil cracking in the microactivity test (MAT). It has been found that the ratio C3/C4 is a constant for zeolite Y-containing commercial FCC catalysts with a unit-cell size between 2.423 nm and 2.433 nm and so for framework Si/Al > 10 the electric field gradient in zeolite Y is constant. High coke on catalyst and contaminant nickel had no influence on the C3/C4 ratio. In the presence of contaminant vanadium the C3/C4 ratio depends on the activity of the catalyst; with decreasing activity the C3/C4 ratio increases. Mixtures of steamed zeolite Y- containing FCC catalysts and ZSM-5 containing additives showed much higher C3/C4 ratios, i.e. the electric field gradient in ZSM-5 is much higher than in zeolite Y.