Mixing of two liquids in a new multifunctional channel reactor developed by AlfaLaval has been studied both experimentally and through computational fluid dynamics (CFD). As the channels are quite narrow the Reynolds numbers are low and the bulk of the channel is within the turbulent boundary layer. This makes accurate a priori predictions of the flowfield difficult and experimental validation necessary. Particle image velocimetry (PIV) was used to measure the flowfield, whereas planar laser-induced fluorescence was used for a scalar concentration field. CFD simulations were performed with the commercial software Fluent 5.5. Different turbulence models were tested and compared with PIV The best predictions were obtained with a low Reynolds boundary layer k-epsilon turbulence model. Mixing of a passive tracer, including mean concentration and concentration variance, was calculated with the turbulent mixer model of Baldyga. A reactive system with diazo coupling between 1-naphthols, 2-naphthols and diazotized sulphanilic acid was studied both experimentally and theoretically due to its sensitivity to mixing conditions. The interpolation model of Baldyga was used to predict the evolution of the species. Good agreement was found between simulations and experiments for both the flow field and the reactive system.