The modeling for the pumpdown of a vacuum chamber with a reversibly adsorbed phase is carried out and the outgassing equation which predicts variations of surface coverage and gas density with time is derived. When it is assumed that the wall surface of a vacuum chamber consists of oxide layer and this layer involves weakly chemisorbed (nondissociate) H2O with desorption energies from 20 to 30 kcal/mol, it is shown that the calculated outgassing rate from the outgassing equation well explains the experimental result that the outgassing rate measured by the orifice pumping method in the unbaked stainless steel chamber is dependent on pumping speed.