A non-isothermal riser model for an fluid catalytic cracking (FCC) riser-reactor has been proposed. The model takes into account the volume expansion owing to cracking reactions and also the temperature drop because of the heat of endothermic reactions. The heats of cracking reactions were estimated by taking the macroscopic difference of enthalpies of products and reactants along the length of the riser. A model has also been developed to calculate the heat transfer mixing height in the riser to verify the assumption of instant thermal equilibrium between the feed and the catalyst. The models were incorporated in CATCRAK [Ind. Eng. Chem. Res. 34 (1995) 3737] and temperature and conversion profiles within the riser were obtained. A temperature drop of about 30-40 degreesC (typical of commercial applications) was predicted. The model can be used in finding optimum temperature schemes for the existing FCC systems as well as to suggest an optimum riser length for new systems to improve gasoline yields.