The trends to the formation of gum precursors in gasoline were studied through the yields and selectivities of C5 olefins in the conversion of both a VGO and nC16 over two equilibrium commercial FCC catalysts. The experiments were performed at 500 and 550 degreesC, catalyst-to-oil ratio of 6.1 and short contact times of up to 12 s in a Riser Simulator laboratory reactor. The behaviors of C5 olefins observed in the conversion of both reactants were similar: the yield of olefins (linear, branched, and cyclic) and dienes increased as a function of contact time and temperature, the amounts being different according to the characteristics of the catalysts. Gasoline composition was very sensitive to the formulation of the catalysts and their resulting hydrogen transfer capabilities. In the conversion of gas oil, maximum selectivities of about 59, 21, 31, and 3.3% were observed for total, linear, iso-, and cyclic olefins in the C5 group, respectively, and the maximum selectivity of dienes in the group was 3.9%. For the same hydrocarbons, the conversion of nC16 showed higher yields at the same conditions. It was verified that the reaction temperature is especially important to control the yield of diene compounds. A simple test methodology to predict the stability of FCC naphthas could be based on a reduced set of experiments with equilibrium catalysts, either with a commercial feedstock or test reactant nC16.