Phase separation behavior associated with ultrathin films of model polyurethanes has been studied by reflectance infrared spectroscopy. Our data clearly demonstrate that the phase separation behavior and the domain size of polyurethanes in the ultrathin film region differ significantly from its bulk state. The kinetics measured are considerably slower than the bulk state. In addition, the degree of phase separation is also highly dependent on film thickness. The phase separation obeys an Avrami type rate process. Using a Monte Carlo simulation method to fix the location and orientation of the nucleating points, the phase separation behavior can be modeled. The relatively slow rate of phase separation, the ultimate degree of phase separation, and the observed high value of chain orientation on the substrate are consequences of having a polyurethane with semirigid hard segments.