The rate and the catalyst decay constants used in the three-lump kinetic model proposed by Weekman (Ind. Eng. Chem. Prod. Res. Dev. 1968, 7, 90-95) for the catalytic cracking reactions were correlated as a function of the total sulfur and nitrogen contents and carbon distribution evaluated by the n-d-M method (ASTM D-3238-82 Method, 1982) using information reported in the literature about the cracking of 16 different feedstocks. The correlations developed in this work are more accurate than those previously reported. Catalytic cracking in a microactivity reactor of six gas oils was used to determine the effect of feedstock properties on FCC product selectivities. It was found that the sulfur content has a strong influence on-kinetic constants and the effect of nitrogen is lower. Good predictions of gasoline yields were obtained from the kinetic model, with an average absolute deviation of less than 1.5% with respect to experimental data.