The flow-field in a Reaction Injection Moulding, RIM, machine mixing chamber was characterized using the Particle Image Velocimetry (PIV) technique. This study covered the industrial range of Reynolds numbers, Re, for the RIM process, from 100 to 500, setting the overall trends of the flow-field in that range, with particular focus on the flow regime transition range of 100 <= Re <= 150. The instantaneous 2D velocity vector maps were obtained with PIV in a plane containing the chamber and injectors axis. From instantaneous velocity data, the following quantities were computed: the probability density functions, the average velocity, and turbulence intensity. The present study sheds additional light into the transition of flow-field regime that marks the onset of mixing on RIM, and also into the mixing mechanisms and respective underlying flow-field features.