The mixing performance of a batch stirred tank with four Ekato Intermig(R) impellers is investigated in this paper by experimental and computational methods. We considered three impeller speeds corresponding to Reynolds numbers 37, 50 and 100, all in the laminar regime. For the purposes of model development and flow validation, Newtonian rheology is assumed, where the fluid density and viscosity is set to 1247.6 kg/m(3) and 0.4 kg/(m s), respectively. The computed velocity field and mixing patterns are validated using Particle Image Velocimetry, acid-base visualization experiments and Planar Laser-Induced Fluorescence. All three techniques reveal excellent agreements between the experiments and computations. Also, detailed Lagrangian analysis of mixing, using particle tracking and stretching simulations, is presented for two flowrates in the laminar regime. It is shown that severe compartmentalization exists in the vessel and transport in the axial direction is very slow. Characterization of local micromixing intensities is presented by computing the distribution of intermaterial area density and striation thickness distribution (STD) from the stretching field. It is found that the STDs at both flowrates are identical despite significant differences in the stretching field, suggesting that at low stirring rates micromixing performance is independent of agitation speed. (C) 2004 Published by Elsevier Ltd.