Results are presented and discussed of a laboratory study that measures T-peel interply adhesion of polymer modified bituminous membranes assembled using typical field application techniques and a wide range of asphalt temperatures. This evaluation includes both standard and polymer modified asphalts, and additionally examines factors such as: a) interply thickness, b) tendencies for the inclusion of blisters, and c) the effects of dark oven heat aging at 70°C (158°F) of the completed assemblies. The findings indicate that the use of elevated asphalt application temperatures, conventionally proposed to improve adhesion, has little to no effect on interply T-peel strengths. The results also illustrate patterns of interply thickness as it relates to asphalt application temperature and/or application technique. Additional indications reveal a tendency to develop blisters that are dependent on application technique and asphalt temperature. The study also proposes that if interply adhesion is considered to be a primary criterion for improved performance of polymer modified bituminous sheet systems, the use of polymer modified interply asphalts must be given due consideration for use in this application.