The forces of interaction between a flat polytetrafluoroethylene (PTFE) surface and an a-alumina or amorphous silica sphere in cyclohexane as a function of distance of separation have been measured using the colloid probe technique in an atomic force microscope in a liquid cell. Repulsive van der Waals forces were predicted for the forces of interaction between PTFE and a-alumina or silica according to the negative Hamaker constants, which were calculated from the dielectric response functions of materials using the Lifshitz theory. The measured forces fit well to theoretically calculated forces, which include the retardation contribution. The retardation effects are apparent: at large distance of separation (>4-5 nm), where the measured forces are weaker than the nonretarded prediction.