The nature of phase mixing in semi-interpenetrating polymer networks (SIPNs) of poly(vinyl chloride) (PVC)/thermoset blends was studied by using both the glass transition temperature third power composition equation and DMTA modeling. From 5% to 15% by weight of an oligomeric MDI isocyanate or a low viscosity epoxy were blended separately with WC to make two series of SIPNs. The DMTA-derived T-g data were modeled by the T-g third power composition equation to characterize "energy interaction" features of PVC/thermoset phase mixing. Fitting experimental T-g Values gave estimations of the characteristic parameters, K-1 and K-2 of the T-g power equation. K-1 and K-2 were each positive for the PVC/epoxy (K-1 = 1.1, K-2 = 7.2) and the PVC/isocyanate (K-1 = 29.9, K-2 = 38) blends, showing that binary hetero-interactions (enthalpic effects) and conformational redistributions (entropic effects) during the binary hetero-interactions both contributed favorably to phase mixing. Negative K-1 - K-2 values for both groups of blends indicate entropic contributions predominate. The thermoset dilution produced a lightly crosslinked thermoset network, which was locked into the amorphous WC, forming a mixed thermoset/PVC SIPN domain. Conformational redistributions of WC and thermoset segments continue to occur within the developing SIPN domain before phase separation can occur. The combined series-parallel Takayanagi coupling model, which assumed that a WC phase and a mixed PVC/thermoset SIPN phase coexisted, was employed to describe the viscoelastic behavior of PVC/thermoset blends, Reasonable fits between the experimental DMTA plots and modeling predictions were found. The predictions were not sensitive to the degree of series-parallel coupling. The modeled DMTA plots, like the experimental results, exhibited only a single E " peak in the glass transition temperature range for both the PVC/PAPI and PVC/epoxy systems. Thus, very small PVC/thermoset SIPN domains (< 20 nm diameter) that are dispersed in separate domains of a WC-rich phase (despersive phase mixing characteristics) provide a reasonable explanation of the blends' phase structures.