The functioning of the riser of catalytic cracking units was simulated using a new kinetic model of the reactions and an accurate description of gas and catalyst hydrodynamics in the riser section. The kinetic model is based on a molecular description of cracking and hydrogen transfer reactions. It may be seen as an intermediate model between simple lumping of cuts and the ’single events’ method. Chemical lumps, such as alkanes, alkenes, cycloalkanes, alkenic cycloalkanes and aromatics, are defined in different cuts. Kinetic constants are determined from experimental results obtained in a laboratory scale tubular reactor (M.A.T). The data on gas and solid flow in the riser are assessed by various experimental techniques : isokinetic sampling. Pitot probe, gamma-ray tomography and tracing in a cold set-up or in industrial risers. The model results were compared with industrial yields.