A micromechanical model for the onset of shear thickening in polymer stabilized colloidal dispersions is derived. The model incorporates both the stabilizing forces of the polymer brush or " coat" and the associated modification of the hydrodynamic interactions. Comparison is made to simulations and experiments on well -characterized dispersions. The model is shown to provide, a quantitative prediction for the onset of shear thickening in polymer coated colloidal dispersions as compared to simulations. For comparison with experiments, where independent characterization of some parameters is not available, the onset of shear thickening can be predicted with realistic assumptions for the model parameters. The model quantifies the sensitivity of the shear thickening transition to both the interaction potential and the hydrodynamic permeability of the polymer coat. (c) 2005 The Society of Rheology.