Microreactors are gaining increasing importance in chemical engineering, since their large area to volume ratio leads to a high efficiency with respect to heat and mass transport. This in turn results in better selectivity and yield for chemical reactions. The mixing of two liquid feed streams in T-shaped micromixers is investigated experimentally and by means of numerical simulations. Measurements of flow fields and concentration profiles are done with Particle Image Velocimetry and Laser Induced Fluorescence measurement techniques, respectively. Numerical tracer experiments are performed and used to evaluate the quality of mixing. Keywords: micro mixing, Laser Induced Fluorescence, Particle Image Velocimetry, Direct Numerical Simulation.