In this work, a kinetic model for hydrocracking of heavy oils is proposed. The model includes five lumps: unconverted resid (538 degrees C+), vacuum gas oil (VGO; 343-538 degrees C), distillates (204-343 degrees C), naphtha (IBP-204 degrees C), and gases. Kinetic parameters were estimated from the experimental results obtained in a fixed-bed downflow reactor. The proposed lump kinetic model fitted the data for hydrocracking of Maya heavy crude with a Ni/Mo catalyst at 380-420 degrees C reaction temperature, 0.33-1.5 h(-1) liquid hourly space velocity, 5000 scf/bbl, and 70 kg/cm(2). Only resid cracked to naphtha at 380 degrees C, but all heavier fractions produced some naphtha at higher temperature. The predicted product composition is in good agreement with experimental values with an average absolute error of less than 5%.