For especially highly concentrated suspensions, slip at the wall is the controlling phenomenon of their rheological behavior. Upon correction for slip at the wall, concentrated suspensions were observed to have non-Newtonian behavior. In this study, to determine the true rheological behavior of model concentrated suspensions, "multiple gap separation method" was applied using a parallel-disk rheometer. The model suspensions studied were polymethyl methacrylate particles having average particle sizes, in the range of 37-231 mu m, in hydroxyl terminated polybutadiene. The effects of particle size and solid particle volume fraction on the wall slip and the true viscosity of model concentrated suspensions were investigated. It is observed that, as the volume fraction of particles increased, the wall slip velocity and the viscosity corrected for slip effects also increased. In addition, for model suspensions in which the solid volume fraction was >= 81% of the maximum packing fraction, non-Newtonian behavior was observed upon wall slip correction. On the other hand, as the particle size increased, the wall slip velocity was observed to increase and the true viscosity was observed to decrease. (c) 2005 Wiley Periodicals, Inc.