The oxygen atoms in a dry film of a copolymer of tetrafluoroethylene (97 mol %) and perfluoropropyl vinyl ether (3 mol %) are hardly detectable by ESCA because the atomic percent of oxygen in the copolymer is small and oxygen atoms are not likely to be found in the outer region of a dry surface. When the film is immersed in liquid water, however, a conspicuous O-1s peak appears on the ESCA spectrum, and its intensity increases with the immersion time. The advancing and receding contact angles measured by the sessile droplet method deviate from those for a dry film progressively with water immersion time. When the surface tension of water is lowered by adding a small amount (0.01%) of a surfactant (Scourol 400) or ethanol (50 vol %), the surface configuration change decreases according to the decrease in the surface tension of the liquid. The maximum extent of the surface configuration change is correlated to the differential surface tension (the difference between the solid surface tension and the liquid surface tension). The greater the differential surface tension, the larger the extent of surface configuration change that occurs to minimize the interfacial tension.