The interactions between silica and mica surfaces bearing an adsorbed cationic polyelectrolyte, a highly quaternarized poly(2-vinyl pyridine), have been measured using an atomic force microscope with tips modified to incorporate a silica colloid probe. The interactions exhibit classical DLVO behavior, with no steric component detectable at low electrolyte concentration. The diffuse layer potentials of these "composite" interfaces appears to be a function of both the adsorbed layer and underlying surface charge. The point of zero charge of the surfaces differs greatly from the isoelectric point of the unmodified surface, but trends in pH versus surface potential behavior are similar. Diffuse layer potentials derived using DLVO analysis successfully predict the dissimilar electrostatic interactions measured between coated and uncoated surfaces. Charge reversal due to dissimilar double layer overlap has also been clearly demonstrated for the first time.