Three types of urethane-acrylate IPN’s, two sequential and one simultaneous, were prepared in which the urethane was formed under thermal polymerization; and the acrylate, by photopolymerization. The IPN morphologies were studied by transmission electron microscopy (TEM) and dynamic mechanical analysis (DMA). When the urethane was reacted to relatively high conversion prior to photoinitiation of the acrylate, the IPN’s obtained were characterized by very small domain sizes (<20 nm) and a single T-g between those of the two components. When the acrylate polymerization was induced following much smaller conversion of the urethane, or if the acrylate was polymerized first, domains of larger size (>100 nm) were obtained. In these two instances, if the T-g’s of the individual components are well separated, dynamic mechanical analysis indicated separate T-g values close to the T-g’s of the individual components. The degree of phase mixing in each IPN was estimated from the T-g values from the Fox equation, and these correlate with the various processing conditions.