In this study, the reaction kinetics of thermoplastic polyurethane (TPU) polymerization in situ with poly(vinyl chloride) (PVC) was characterized with differential scanning calorimetry. The presence of PVC appeared to enhance the thermodynamic drive for TPU hard segment phase separation and domain ordering from the PVC/TPU reactant mixture. Prior to hard segment phase separation, and thus some critical conversion, TPU polymerization in situ with PVC obeyed the same nth order, phenomenological kinetic rate law followed by neat TPU polymerization. In addition, the overall rate constant employed in the rate law increased in the presence of PVC. After hard segment phase separation, and the resulting physical cross-linking of the PVC/TPU reactant mixture, the reaction kinetics of TPU polymerization in situ with PVC became diffusion controlled. © 2005 Elsevier Ltd. All rights reserved.