Composites of poly(methyl methacrylate) and various nanoscale inorganic particles (zinc oxide, titanium dioxide, zirconium dioxide, silicon dioxide, and aluminum nitride) were prepared by in-situ bulk polymerization using 2,2'-azobis(isobutyronitrile) as initiator. The particles of ZnO, TiO2, and ZrO2 were surface-modified by alkylphosphonic acids to render them dispersible in the monomer. The effect of these nanoparticles on the free radical polymerization was investigated. Regardless of chemical nature and size, the particles suppress the autoacceleration which would otherwise occur in the bulk free-radical polymerization of methyl methacrylate (MMA). A degenerative chain transfer is proposed to take place between surface-adsorbed water on the particles and propagating chain radicals. This reaction competes with normal termination. Formation of vinylidene chains ends originating from disproportionation is suppressed. In consequence, thermal stability of PMMA produced in the presence of particles is improved. Aggregation of individual particles upon polymerization has been observed and presumably is due to interparticle depletion attraction, even though the particles are individually dispersed in the monomer. Formation of particle clusters is suppressed when a difunctional monomer (e.g., ethylene glycol dimethacrylate) is used as comonomer. The cross-linked medium slows down the diffusion of the particles and therefore interferes with particle aggregation via a depletion mechanism.