The rheology and microstructure of 44 rim diameter silica particles suspendec. in entangled poly(ethylene oxide) (PEO) melts ire studied through measurement. of filled melt viscosity and X-ray scattering measurement of interparticle structure factors, S(q,phi), where q is the scattering vector and phi is the silica volume fraction The particles have a similar refractive index to PEO which minimizes van der Waals attractions acting between particles. The introduction of particles causes an elevation in the viscosity of the nanocomposite melt more than would be expected of particles merely interacting with hard core repulsions Further addition of particles causes I rise in the elastic and viscous mod Lift The rheological characterization of these nanocomposite melts is discussed in terms of several critical particle volume fractions that result from confinement of polymer. adsorption of polymer segments to the particle surface, and overlap and entanglement. of adsorbed polymer as the particle volume fraction is increased. Characterization of the particle microstructure shows that the association of the Polymer with the particles drives the particles to structure more than would be expected of particles with interactions governed merely by hard core repulsions show signs of instability in the polymer melt at a common elevated volume fraction independent of polymer molecular weight.