This study shows the ability of the riser simulator, a novel internal fluidized bed, to determine the heats of gas oil cracking. With this goal, a gas oil feedstock is cracked using both Octacat and GX-30 catalysts. The experimental conditions studied include a catalyst-to-oil ratio of 4.85, typical reaction times of 3, 5, and 7 s, and temperatures in the 500-550 degrees C range. The heats of cracking are calculated from stoichiometric considerations and following a methodology consistent with classical thermodynamics, Stoichiometric coefficients for the product spectrum are estimated from the GC-FID chromatogram of the cracked products, The significance of estimations of the heats of cracking is demonstrated, in the present study, using a model for the FCC plant. These calculations are based on overall enthalpy balances applicable to the riser cracker and to the catalyst regenerator.