The relationship between the mass uptake and the swelling of arachidic acid Langmuir-Blodgett (LB) films exposed to water vapor was investigated, Combining sorption data from the quartz crystal microbalance with swelling data from the surface forces apparatus (SFA), it was found that films exposed to water vapor (approximate to 75% RH) absorb 0.22 moles water/mole LB film with an associated swelling of 0.2 Angstrom/film layer, This degree of film swelling is less than that predicted if the water taken up exhibits the molar volume of bulk liquid water. The configuration of the films in the SFA, where they are trapped between impermeable surfaces, makes possible the measurement of the lateral diffusion coefficient of water in these layered materials. This was found to be 1.5 x 10(-10) cm(2)/s, which is at least 100 times faster than diffusion normal to the layers as measured with the microbalance.