The photopolymerizable semi-interpenetrating polymer network-II being studied consists of a linear copolyester, a crosslinked network of multifunctional acrylate monomers and a very efficient photoinitiator system. Using both Raman and dynamic mechanical spectroscopy (DMS), it has been determined that the polymerizing network vitrifies after a very low level of crosslinking is attained. This results in a sample that has a Tg which is substantially lower than would be expected from knowledge of the values of Tg for the individual components of the blend. The extent of reaction and, therefore, the glass transition temperature can be increased by heating the sample during photopolymerization. In addition, the glass transition temperature of the network can be increased by thermally curing the sample after photopolymerization. It has been found that once the network has been fully cured, the glass transition temperature is much higher than would be predicted. This has been interpreted in terms of synergistic effects; conceivably, the acrylates form a tight matrix around the linear copolyester and effectively reduce the molecular mobility of the components. The results also show that the acrylates and the copolyester are not inherently miscible. However, if a sufficient concentration of acrylates is present, it is possible to prevent the copolyester from phase separating. Dynamic mechanical spectroscopy data do indicate that these semi-inter-penetrating polymer networks-II (semi-IPN-II) samples are somewhat heterogeneous; however, there does not seem to be any gross phase separation. Transmission, electron microscopy (TEM) results show that the most fully cured sample has domains on the order of 50–250 Å associated with the copolyester phase.