We perform a thermodynamic analysis of the polymerization-induced phase separation in nanoparticle-monomer-polymer blends using a simple model recently proposed by Ginzburg (Macromolecules 2005, 38, 2362). The model was adapted for ternary blends constituted by nanoparticles, a monomer and a linear polymer, where the relative fractions of polymer and monomer determine the conversion in the polymerization reaction. The analysis showed that phase separation can occur in the course of polymerization for a large range of values of the relevant parameters (interaction parameter and polymer and particle sizes). This possibility has to be considered when the intention is to fix a uniform dispersion of nanoparticles in a monomer through its polymerization.