A model is developed that predicts the probability of conversion of gas molecules undergoing catalytic reactions in a fluidized bed reactor. The model takes into account the stochastic axial movement of a gas molecule, as it alternates in visits between bubble and emulsion in a two-phase flow or between bubble, cloud and emulsion in a three-phase flow, and its catalytic reaction in the cloud and emulsion. From the residence time distribution of a molecule and its reaction kinetics, one obtains the probability of conversion.