Rheological investigations of blends of PVC (polyvinyl chloride) and DAOP (diallyl orthophthalate) with and without free radical initiators have been conducted under steady and dynamic shearing modes. The results showed that the added DAOP acted as a reactive plasticizer for improving PVC processing, thus greatly decreasing the risk of the decomposition of PVC. The viscosity of the uncured blends was also significantly reduced and obeyed the log-additivity rule with the weight percentage of DAOP. Dynamic rheology of some of the uncured blends indicated the physical gel-like behaviour due to the presence of small crystallites acting as physical crosslinks. Blends with low levels of PVC obeyed the Winter-Chambon gelation criteria and the viscosity diverged at the gel point during the DAOP polymerization, but when the PVC level exceeded 20 wt% the tan delta did not exhibit a frequency independence and the viscosity remained finite up to the end of the reaction. This unusual behaviour is consistent with heterogeneous gelation due to either phase separation between DAOP and PVC components or the non-fractal nature of the gel structure. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.