Introducing hard spherical polysilicate nanoparticles up 10 0.30 volume fraction into narrow molecular weight (M) fractions of poly(dimethylsiloxane) (PDMS) that spanned its unentangled and entangled polymer chain regimes produced transparent and colorless polymer nanocomposites (PNCs) with zero-shear-rate viscosities eta(0) either greater or less than the polymer matrix. Below the PDMS critical molecular weight for chain entanglements to start influencing eta(0) (M-c), nanoparticle inclusion increased the PNC eta(0) consistent with the reinforcement mechanism from traditional fillers of much larger particle sizes. Conversely, PNCs using entangled PDMS (M > M-c) exhibited a reduction in eta(0) within a certain concentration range of nanoparticles. The reduction in eta(0) was proposed to be primarily due to the dilution of the entanglement density of polymer chains as evidenced by a shift to higher M-c that was a function of the volume fraction of polymer chains, phi(-0.56)(1), and an increase in the translational motion manifested through an increase in the polymer self-diffusion coefficient. In contrast to the phi(-1)(1) dependence of M-c in concentrated polymer solutions, the dilution effect of the polysilicate nanoparticle on the polymer chain entanglement density was moderated by polymer adsorption, hydrodynamic effects and to some extent by free volume. Above M-c, the PNC eta(0) scaled as phi(2)(M) over bar (3.5)(w) where (M) over bar (w) is the polymer weight-average molecular weight. Nuclear magnetic resonance T-2 spin-spin relaxation measurements found that the increase in polymer mobility due to the nanoparticles became evident only above 2M(c). In contrast to the eta(0) results, the transition to a stronger (M) over bar (w) dependence by 1/T-2 shifted to lower M with increasing nanoparticle concentration.