This paper describes the use of positron emission particle Tracking (PEPT) in the analysis of local particle and fluid velocities in solid-liquid stirred tank reactors agitated with a Rushton turbine and an upward-pumping pitched blade turbine. PEPT captures the full three-dimensional characteristics of hydrodynamics and mixing in stirred vessels, allowing the analysis of the two-phase flow fields. Furthermore, by comparing the liquid and particle velocities, the spatial and temporal variation of the relative particle-liquid velocity can be estimated. Such information reveals considerable heterogeneity in the vessel and facilitates the evaluation of impeller design, particularly with the aim of minimizing mass transfer limitations.