Impingement mixing in a polyether polyol/butanediol/MDI, thermoplastic urethane system was evaluated using measurements of striation thickness distribution, adiabatic temperature rise, and molecular weight. Adiabatic temperature rise was less sensitive to mixing quality for this system than for the crosslinking ones previously studied. The average of the distribution of striation thickness as measured by optical microscopy on the resultant polymer decreased with increasing nozzle Reynolds numbers, Re. Molecular weight increased and its polydispersity decreased with increasing Re. Morphological data indicate better phase separation in the poorly mixed samples. These results are consistent with relatively slow diffusion for the polyol and thus the preferential formation of unattached hard segments at lower Re.