Alumina/poly(methyl methacrylate) (PMMA) nanocomposites were synthesized by an in situ free-radical polymerization process with 38 and 17 nm diameter gamma-alumina nanoparticles. At extremely low filler weight fractions (<1.0 wt % of 38 nm fillers or <0.5 wt % of 17 nm fillers) the glass-transition temperature (T-g) of the nanocomposites drops by 25 C when compared to the neat polymer. Further additions of filler (up to 10 wt %) do not lead to additional T-g, reductions. The thermal behavior is shown to vary with particle size, but this dependence can be normalized with respect to a specific surface area. The nanocomposite T-g phenomenon is hypothesized to be because of nonadhering nanoparticles that serve as templates for a porous system with many internal interfaces that break up the percolating structure of dynamically heterogeneous domains recently suggested to be responsible for the T-g reductions in polymer ultrathin films [Long, D.; Lequeux, F. Eur Phys J E 2001, 4, 371]. The results also point to a far field effect of the nanoparticle surface on the bulk matrix.