Semi-1 and semi-2 interpenetrating polymer networks (IPNs) of poly(vinyl chloride) (PVC) and in situ formed poly(ethyl acrylate) (PEA) have been synthesized using diallyl phthalate and ethylene glycol dimethacrylate as the crosslinkers of PVC and PEA, respectively. These two types of IPNs have been compared with respect to their physical, mechanical, and thermal properties and an endeavor has been made to find a correlation of these properties with the morphology generated in these systems. The semi-1 IPNs displayed a decrease in their tensile strength and modulus while in contrast; the semi-2 IPNs exhibited a marginal increase with increasing crosslinked PEA incorporation. The semi-l and semi-2 IPNs containing 10 and 30 wt % of PEA displayed a two-stage degradation typical of PVC in their thermogravimetric and DSC studies while confirming the increased stability of the samples with higher percentages of PEA. The softening characteristics as detected by the extent of penetration of the thermomechanical probe as has been detected by thermomechandica; analysis are in conformity with their mechanicals. The biphasic cocontinuous systems as explicit from the morphological studies reveal fibrillar characteristics in both the systems. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3450-3459, 2009