The flow and mass transport within a twice-folded microchannel is investigated. For the measurement of the concentration field the microlaser induced fluorescence technique (mu LIF) is engaged. Microparticle image velocimetry (mu PIV) is further employed to obtain the velocity field. Deviation from standard mu PIV by seeding a partial stream of the flow only, results in partial velocity profiles. This procedure allows for the observation of the contact interface between the seeded and the unseeded liquid. Distorted contact interfaces is recognized within and after each bend, indicating strong secondary flows. The measurements are compared to numerical simulations, solving for both the flow, and the mass-transport equations, and, subsequently, performing particle tracking. The results are, on one hand, in reasonable agreement with the experiments. On the other hand, the differences between the contact interface from the mass-transport equation and from particle tracking clarifies the effect of diffusion. (c) 2007 American Institute of Chemical Engineers.