Wereport a small angle neutron scattering study of the thermodynamics of a polymer mixture in the presence of nanoparticles, both in equilibrium and during phase separation. Neutron cloud point measurements and random phase approximation (RPA) analysis demonstrate that 1-2 mass % of C60 fullerenes destabilizes a highly interacting mixture of poly(tetramethyl bisphenol A polycarbonate) and deuterated polystyrene (TMPC/PSd). We unequivocally corroborate these findings with time-resolved temperature jump experiments that, in identical conditions, result in phase separation for the nanocomposite and stability for the neat polymer mixture. At lower C 60 loadings (viz. 0.2-0.5 mass %), stabilization of the mixture is observed. The nonmonotonic variation of the spinodal temperature with fullerene addition suggests a competitive interplay of asymmetric component interactions and nanoparticle dispersion. The stability line shift depends critically on particle dispersion and vanishes upon nanoparticle agglomeration. © 2010 American Chemical Society.