Water vapor absorption and desorption by poly (phenylene oxide) (PPO) and sulfonated PPO (SPPO) membranes were studied at a constant temperature of 30-degrees-C and over a broad range of water activity (0.05 less-than-or-equal-to a < 0.8) by the weighing method. The experimental sorption isotherms of both PPO and SPPO possessed a general sigmoidal shape, which suggested that they belong to type II; thus, the data may be quantitatively analyzed according to the BET or GAB equation for multilayer sorption processes. The number of site-bound water molecules per monomeric unit was increased by a factor of 150 after sulfonation of PPO. The features of the reduced absorption and desorption curves of both PPO and SPPO suggested that the sorption processes were non-Fickian. The diffusion coefficient calculated from the slope of the initial linear part of the curves showed concentration dependence. The permeability of water vapor through SPPO was more pressure-dependent than was that through PPO. The Flory-Huggins interaction parameter derived from experimental data on SPPO had a smaller value compared with that of PPO and was a monotonic increasing function of water activity in the low-activity region and then leveled off at a > 0.6, showing a corresponding initial decrease of the polymer-water interaction, which then gradually reached a certain stable value. Water clustering for SPPO was much less than that for PPO, which is clear proof of its higher hydrophilicity. The results from this study showed that SPPO could be an excellent dehumidification membrane material.