The glass transition temperature of semi-interpenetrating polymer networks (semi-IPNs) of atactic polystyrene (PS) in crosslinked methacrylates was studied by systematic variation of the morphology, that is domain size, continuity and concentration in the domains. Semi-IPNs were prepared from homogeneous polymer solutions in a two-step process, of liquid-liquid demixing upon cooling followed by gelation, and subsequent ultra-violet polymerization. Semi-IPNs with very little developed phase separation could be obtained, which exhibited optical transparency and a mechanical behaviour similar to that expected for corresponding homogeneous systems. A low PS concentration inside the domains indicated that these domains consisted of only an interphase layer. Samples that were phase separated to a larger extent showed normal two-phase behaviour.