The permeability constants of both liquid water and water vapour through eleven polymer films have been measured at three temperatures by weighing method．The per-meation rate of liquid water in polyethylene，polycaprolactam，and polyethylene—tereph-thalate decreases gradually and approaches a limiting value owing to a change of structure in the polymer under the action of liquid water．The permeability constants of amor-phous polymers for both liquid water and saturated vapour are nearly the same at a given temperature，but the ratio of them increases slightly above unity with increasing temperature．In the case of crystalline polymers，the permeability constants for liquid water are several times greater than those for saturated vapour，suggesting the existence of viscous flow of the liquid water in the film．