An experimental investigation of a turbulent air-water bubbly flow in a plane vertical mixing layer is presented. An important characteristic of this bubbly flow is that the slip velocity is of the same order of magnitude as the liquid velocity. For various inlet conditions of liquid velocity and void fraction, the distributions of velocity, turbulence intensity and void fraction were measured. The mean velocity fields in both phases present self-similar evolutions that preserve the main characteristics of single-phase mixing layer. The spreading rate of the mixing layer is found to be significantly greater in bubbly flow than in single-phase flow. Moreover, the global structure of the flow proved to be sensitive to void fraction contrast at the inlet. The void fraction distributions exhibit a pronounced peak in the wake of the splitter plate. This peak decreases in intensity and is displaced significantly in the lateral direction. The turbulence intensity is also greater in bubbly mixing layers : it is shown that this effect is related to the bubble drift velocity. A heuristic model for the separation of bubble and shear-induced velocity fluctuations is presented and discussed. Finally, it is observed that the RMS velocity of the gas phase results from combined effects of turbulent dispersion and self-induced fluctuating motions of the bubbles.