We report partition coefficients (K) and mutual diffusion coefficients (D12) of a homologous series of n-alkyl acetates as a function of particle loading in poly(methyl acrylate)/silica nanoparticle composites. The nanocomposites, prepared using nearly monodisperse poly(methyl acrylate) (PMA, Mw ≃ 56 kDa) and spherical silica (SiO2) nanoparticles of two different diameters (d ≃ 14 nm and ≃50 nm), are all well above their glass transition temperatures. The K values indicate that the addition of silica causes sorption in excess of that expected from a simple blocking effect. Also, for the three smallest penetrants in the series, the addition of nanofiller suppressed the D12, but hardly affected the diffusant concentration dependence of the diffusivity. Conventional composite theory can account quantitatively for the suppression of D12 with particle volume fraction, ψP, for nanocomposites with the larger nanoparticles (d ≃ 50 nm) but not for those made with the smaller nanoparticles (d ≃ 14 nm), which showed a much stronger effect. Further, the addition of the smaller nanoparticles to PMA also notably altered the concentration dependence of D12 for the largest alkyl acetate studied, n-butyl acetate. This implies changes in selectivity with filler content for the composites made with the smaller nanoparticles, an effect not seen with the larger particles. Overall the results suggest an interfacial layer proximal to each nanoparticle with modified transport characteristics, whose influence on the average properties is much more obvious in composites made with smaller particles.