The rheological behavior of two hyperbranched polymer/silica suspensions with different dispersion states, surface chemistries, and concentrations of the silica nanoparticles was investigated in terms of viscoelastic properties, activation energy for viscous flow, and yield stress. The viscoelastic properties of both types of suspensions were reduced to a master curve that was a function of the limiting viscosity and shear modulus. A liquid-to-solid transition and correlated activation energy change were found to occur for particle volume fraction in the range of 5-10% for well-dispersed systems and 20-25% for systems where silylated particles were agglomerated. The viscosity of the suspensions was found to be considerably higher than that predicted by the classical percolation model for concentrated particle suspensions; this was argued to result from an immobilized layer of polymer on the surface of the silica particles. The percolation model was therefore modified to include such confined layer in order to predict the viscosity as a function of filler fraction. In the case of silylated particles with weak interactions with the polymer, the model based on an immobilized layer of thickness in the range of 2-5 nm reproduced the data. In the case of well-dispersed particles with strong interfacial interactions, the immobilized layer was correlated to the average distance between adjacent particles. In this case the model predicted an exponential increase of the viscosity with particle fraction and that the whole matrix gelled at particle concentrations larger than 5 vol %, corresponding to a 7.5 nm thick immobilized layer. © 2010 American Chemical Society.