We report 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 C-60 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