The surface forces between a Si3N4 tip and sample surfaces of various oxides, such as SiO2, SnO2, and Al2O3 were measured in electrolytes with various pH values using atomic force microscopy. The isoelectric points were estimated from the force-distance curves. A similar treatment was applied to hydrocarbon-modified quartz surfaces. Almost no effect of the surface modification was found in neutral solutions, while a small difference in the force-distance curves before and after the modification was observed in acidic and alkaline solutions. The electric double layer force due to the concentration profile of the counterions at the oxide/water interface is discussed in terms of acid-base reactions of -OH groups on the oxide surface.