Solid-liquid mass transfer coefficients were determined in a stirred reactor for the hydrogenation of 4-nitrobenzoic acid. The effects of the catalyst particle physical properties were investigated and the findings compared to the model system of aqueous dissolution of salicylic acid. In an attempt to explain these mass transfer observations the technique of positron emission particle tracking (PEPT) has been used to study the effect of different operating conditions on the liquid flow patterns and, in particular, the particle-liquid slip velocities. Time-averaged slip velocities were determined, thereby revealing information about particle-liquid behaviour and zones of poor mass transfer in agitated reactors.