We used the colloidal probe technique with the atomic force microscope (AFM), to study the interactions between a tungsten oxide sphere, WO3, and flat oxidized tungsten and cobalt surfaces in aqueous electrolytes. We have investigated the effects of adsorption of cobalt ions to tungsten oxide surfaces and adsorption of polyethyleneimine (PEI). Low concentrations of cobalt ions to a WO3system resulted in extended hydration forces and lower absolute value of the surface potential. It was shown that PEI adsorbs to the WO3 surfaces and induces an electrosteric repulsion in both the symmetric (WO3-WO3) and asymmetric (WO3-CoOOH) system. The possible complexation of cobalt ions with PEI does not significantly influence the thickness of the adsorbed layer