We Studied theoretically the effect of grafted polymer layers on electro-osmotic solvent flows. This allowed us to predict qualitative features of the electrophoretic mobility of charged particles on which small polymers (charged or neutral) have been grafted. The cases of sparse and dense grafting were both analyzed, accounting for the deformation of the grafted polymers by the electrically induced hydrodynamic flow generated by the charged composite surface. For sparsely grafted polymers, we derived an expression for the modification of the effective electro-osmotic mobility mu (eff) of the polymer-bearing surface in terms of the density and conformation of the grafted chains. Sufficiently strong electric fields modify the conformation of the grafted chains, resulting in a nonmonotonic dependence of mu (eff) on electric field strength. For densely grafted polymers, the electro-osmotic behavior of the system is dominated by the electrophoretic properties of the polymer. In the limit of very high electric fields, the inner region of the polymer layer is deformed by the electro-osmotic flow, resulting in enhanced screening of electro-osmotic flows by the polymer layer.