Diffusion of moisture in poly(p-phenylene terephthalamide) films was observed using a gravimetric method at various vapor pressures at 25 degrees C. The equilibrium moisture regain was 4.9% (g/g) and 4.2% (g/g) at 65% relative humidity for the 16-mu m and 50-mu m thick film, respectively. These values are slightly larger than regular Kevlar or Kevlar 49 that have the same chemical structure of poly(p-phenylene terephthalamide) (PPTA). The sorption and desorption curves were satisfactorily analyzed by the adsorption-controlled diffusion equation assuming reversible bimolecular kinetics. These results strongly indicate that the interaction between the water molecules and amide groups of polymer plays an important role in the diffusion process and has much effect on the shape of sorption/desorption curves. The diffusion coefficients for the films were in the order from 10(-11) to 10(-10) cm(2)/s, which were also larger than those for Kevlar fibers of 10(-12) cm(2)/s. The higher moisture diffusivity in the film reflects sparser molecular packing and existence of interstitial microvoids due to the lower molecular orientation in the noncrystalline region in the PPTA film than in the fiber (Kevlar).