Taylor bubbles correspond to a sequence of gas bubbles surrounded by liquid, which are encountered in many practical applications. Besides their dimensions (e.g. bubble length) and the distance between consecutive Taylor bubbles (i.e. slug length) the main characteristics, rather advantageous, for investigating Taylor bubbles is their reproducibility. So far, most experimental investigations are based on circular cross sections. In addition, most investigations deal with the flow field in the liquid slug only. Little is known about the velocity profile in the liquid film between the bubble and the channel wall. The observations presented here are based on square shaped channels. Moreover, this study is based on a combination of experimental observations and numerical simulations. The experimental setup is vertically oriented and a special injection valve with an extraordinary compensation pipe is tailored to generate steady and reproducible Taylor bubbles. Furthermore, refractive index matching (RIM) is used to avoid refraction caused by curved surfaces. The use of RIM enables very accurate micro Particle Image Velocimetry (mu PIV) observations of the flow fields in the liquid slug and in the lateral liquid film. The mu PIV observations and the numerical simulations both indicate a recirculation flow pattern in the liquid slug. Additionally, both observations indicate a Couette shaped local velocity profile within the liquid film. The no-slip boundary condition is assumed at the gas-liquid interface in the liquid film. This condition leads to the existence of a toroidal circulation within the Taylor bubble, which is similar to the Rybczynski-Hadamard circulation. Both, the experimental observations and the numerical simulations show good agreement for the flow fields in the liquid slug and the liquid film. (C) 2014 Elsevier Ltd. All rights reserved.