Solid-liquid mass transfer coefficients were determined using the technique of dissolving a sparingly soluble solid, salicylic acid loaded onto silica, in water. Mass transfer was found to be dependent on various particle characteristics. Of particular interest is the influence of the particle-liquid density difference. It is suggested that the change in mass transfer coefficient with these parameters is related, to some extent, with the particle-liquid slip velocity. The technique of positron emission particle tracking (PEPT) has been used in parallel with the mass transfer measurements in order to study the effect of different operating conditions on the liquid flow patterns and the particle-liquid slip velocities. Using PEPT, time-averaged slip velocities were determined by simple subtraction of the data from a neutrally buoyant particle. In this way, important information about solid-liquid behaviour and zones of poor mass transfer in stirred vessels is revealed. (C) 2003 Elsevier Science B.V. All rights reserved.