The flow field in a reaction injection molding (RIM) machine mixing head was characterized using the laser Doppler anemometry (LDA) technique. This study covered the range of Reynolds numbers from 50 to 600, normally used in industry for the RIM process. The three time-averaged spatial distributions of velocity components, inside the mixing head, were obtained: the two horizontal components directly from LDA measurements and the third component through numerical integration. Flow-field distributions were also computed, including probability density,functions of the velocity, turbulence intensity, and Reynolds stresses. The present study provides new insights into the mixing mechanisms involved in RIM machines and more generally, in opposed-jet mixers. In particular, new features were found: the dominant role of interactions of the opposed jets as the main mixing mechanism; and that the flow-field dynamics evolve with Re at constant Strouhal number within the typical range, 150 <= Re <= 600, of operation of RIM machines. (c) 2005 American Institute of Chemical Engineers.