The flow fields under laminar conditions in two typical regions of a cavity transfer dynamic mixer consisting of an inner rotor and outer stator were visualized by refractive index (RI)-matched particle image velocimetry (PTV) experiments. The RI of the working liquids and the transparent solid parts of the rotor and stator were matched to allow for unobstructed optical access. The flow fields were predicted by using computational fluid dynamics (CFD) simulations, including models for species transport. Various flow patterns in the dynamic mixer are discussed. The simulated flow fields in the two investigated regions agree well with the experimental data. The effect of gap width between the rotor and stator (sigma) on the scalar mixing was evaluated, and smaller a will achieve better mixing because of the enhanced shearing and higher energy consumption.