Semi-1 and semi-2 interpenetrating polymer networks (IPNs) of poly(vinyl chloride) (PVC and in situ formed poly(butyl methacrylate) (PBMA) have been synthesized using diallyl phthalate and ethylene glycol dimethacrylate as the crosslinkers of PVC and PBMA, respectively. These were then characterized with reference to their mechanical, thermal, and morphological properties. The mechanical and thermal characteristics revealed modification over the unmodified polymeric systems in relation to their phase morphologies. The semi-1 IPNs displayed a decrease in their mechanical parameters of modulus and UTS while semi-2 IPNs exhibited a marginal increase in these two values. The semi-1 IPNs, however, also revealed decrease in the elongation and toughness values away from the normal behavior. The thermomechanical behavior of both the systems is in conformity with their mechanicals in displaying the softening characteristics of the system and stabilization over unmodified PVC. The DSC thermograms are also correlated to these observations along with the heterogeneous phase morphology which is displayed by both the systems especially at higher concentration of PBMA incorporation. (C) 2007 Wiley Periodicals, Inc.