Surface-configuration change due to the change of contacting medium from (dry) air to liquid water and also from (dry) air to a high relative humidity air was investigated for cellulose and cellulose acetate. CF4 plasma treatment was used to tag the surface, and the change of the ESCA F(1s) signal was followed as a function of the (contact) time after the change of contacting phase. In the water immersion mode, fluorine atoms on cellulose (crystallinity 70-75%) decreased to a plateau value (approximately 9% of the dry sample) within 2 min but that on cellulose acetate (degree of substitution 2.45 and amorphous) decreased to approximately 60% in 15 min. The decrease of fluorine atoms was also observed when films were exposed to a high relative humidity but at much slower rates (roughly 1/10000 of the water immersion case). On the exposure of 10 days, cellulose reached a level of approximately 75%, and cellulose acetate roughly 85%. The decline observed for cellulose was found to be linearly proportional to the moisture uptake (weight percent) of film. The data indicated that the swelling of a polymer plays a passive but significant role in the surface-configuration change of hydrophilic polymers when the surface is exposed to high relative humidity or immersed in liquid water.