X-ray photoelectron spectra of C-60 films exposed to air of ca. 20% and 60% humidity at 23 degrees C for 1 h are presented. It was found that the O1s core peak appeared at a binding energy of 533.5 eV for the film exposed to air of ca. 60% humidity, while no peak corresponding to the O1s photoelectrons was observed for the film exposed to air of ca. 20% humidity. The binding energy of 533.5 eV for the O1s core peak is close to that of about 533 eV for the O1s core peak of solid H2O, while it is different from that of 537 eV for the O1s core peak of solid O-2 on C60 film. These facts indicate that the O1s peak observed for the air-exposed C-60 film was due to H2O molecules adsorbed on the him.