We used the colloidal probe technique with atomic force microscopy to study the interactions between a tungsten oxide sphere, WO3, and flat oxidized tungsten and cobalt surfaces in aqueous electrolytes. We investigated the effects of adsorption of cobalt ions to tungsten oxide surfaces and adsorption of polyethyleneimine (PEI). Low concentrations of cobalt ions added to a WO3 system resulted in extended hydration forces and a lowering of the absolute value of the surface potential. PEI was shown to adsorb to the WO3 surfaces and induce an electrosteric repulsion in both the symmetric (WO3-WO3) and asymmetric (WO3-CoOOH) systems. The possible complexation of cobalt ions with PEI does not significantly influence the thickness of the adsorbed layer.