The apparent density of water adsorbed on four different charcoals at 20 °C. has been measured by a method of helium displacement. At low relative pressures the density is appreciably higher than that of liquid water, but falls rapidly to nearly the normal density as the amount adsorbed increases. A very sharp drop in density occurs close to saturation, and is attributed to the blocking of the very small porous spaces through the advent of capillary condensation in this region. It is suggested that the apparent high initial density is due to the size of the molecules of the measuring fluid rather than to the compression of the adsorbed film.The apparent density of the adsorbed water on the desorption branch of the isotherm differs from that on the adsorption branch, which suggests that the mechanisms of adsorption and desorption differ. An attempt has been made to evaluate the constants Vm and C of the multimolecular adsorption theory from one of the adsorption isotherms.