The ability to control the degree of particle dispersion in polymer solutions has been a long-standing subject in colloidal science. While a generally accepted principle is that nonadsorbing polymers can induce depletion attraction, which is mostly temperature independent, the effects of adding adsorbing polymers are still poorly understood. In this study, we investigated the effects of adsorbing polymers on the temperature-dependent stability of nanoparticles. The model systems consisted of silica nanoparticles in low-molecular weight polyethylene glycol) solutions. The detailed microstructures were determined with small-angle X-ray and neutron scattering measurements, while the dynamics of the temperature-dependent microstructures of the nanoparticles and polymers were probed with diffusing-wave spectroscopy. It was found that a poor solvent for polymer could drive adsorbed polymers to leave the particle substrate and return to the bulk solution due to a complicated interaction with surface, while the loss of the steric layer causes the nanoparticles to aggregate at elevated temperatures.