Catalytic cracking of gas oils has been studied in a standard microactivity test (MAT) reactor. The cracking product distribution was measured as a function of temperature. Based on these experimental results, a four-lump kinetic model was developed. Kinetic constants were estimated using the sequential step-optimization method. A nonisothermal nonsteady-state model for a fixed-bed MAT reactor was proposed. The overall heat of the reactions were taken from the macroscopic differences in the enthalpies of the products and reactants. The influence of the feedstocks used and reactor temperature were discussed. The reactor and kinetic model were validated with results from MAT test data. The simulation results are in good agreement with the experimental data.