A mechanistic model for the reaction in a biphasic mixture with a phase transfer catalyst has been developed. The mass transfer of the catalyst between the two reaction phases and the dissolution of the solid in one of the liquid phases are described using the 2-film theory.. The model was applied to the reaction between a sulfonamide and a benzyl bromide in the presence of an aqueous base. The kinetic parameters, partition coefficient of the catalyst and mass transfer coefficients were estimated at different process conditions. Sensitivity analysis was then performed under different conditions using the estimated parameters. These model results were compared with experimental observations and good agreement was obtained.