Measurements of the self-diffusion coefficient of water in NAFION-117 as a function of pressure have been carried out. The natural field gradient of a 7.3 T superconducting magnet was used for the diffusion measurement. The measurements were carried out at 288 K and variable pressure up to 0.25 GPa. The high-pressure diffusion data were obtained for four different water contents between 6.6 and 22 wt %. The calculated activation volume decreased from 13.2 to 2.7 cm(3)/mol as the water content increased from 6.6 to 22 wt %. These results agree with previously published activation volumes extracted from electric conductivity and NMR T-1 measurements. It is suggested that segmental motions within the polymer play a significant role in the transport mechanism for low water content membranes, and that the transport mechanism for high water content membranes is similar to that in liquid bulk water.