A model of water adsorption on metal oxide layers and water outgassing from metal surfaces has been developed. The oxide layer is assumed to have porous structure and a pore length (l) distribution of 1/l2. Numerical evaluation shows that the quantity of water adsorbed is logarithmic with time within a certain time range as experimentally observed. The outgassing rate from surfaces with adsorbed water distributed uniformly on the inner surfaces of individual pores is shown analytically to be inversely proportional to time. This result is consistent with frequently observed pumpdown curves.