The product selectivities of the fluid catalytic cracking (FCC) process are strongly dependent on the properties of the petroleum gas oil reactant. In order to elucidate the complex relationship between gas oil chemical composition and product selectivity, a new technique has been developed which experimentally determines the product distribution of specific gas oil fractions in a realistic chemical environment. This "incremental yield analysis" approach is examined with a representative industrial gas oil. The gas oil is characterized and then divided into boiling point fractions by distillation and into hydrocarbon-type fractions with a chromatographic method. The FCC selectivity of each gas oil fraction is then determined by microactivity testing of blends of each fraction with the original gas oil. Results show that hydrocarbon type is a more significant determinant of the product spectrum than boiling point.