Monte Carlo (MC) and molecular dynamics (MD) simulation techniques have been employed to study the equilibrium and dynamical properties of water vapor sorbed in two different grain boundaries (110/110, 200/200) of a semicrystalline aromatic polyamide, poly(p-phenyleneterephthalamide) (PPTA). Henry’s law constants (H) and isosteric heats of adsorption (q,t) were obtained from Monte Carlo (NVT) simulations, and the influence of polymer mobility on the transport behavior of sorbed water particles was investigated using molecular dynamics (NVE, NVT) simulation techniques. It is shown that the diffusive motion of the water particles is related to a cooperative interaction between the water molecule and polymer conformational dynamics at the surface of the PPTA crystallites. In particular the reorientational motion of the phenyl rings plays a central role in the diffusive mechanism, and an approximate expression for the frequency of the water molecule diffusive jumps was derived in terms of an effective phenyl ring torsional energy barrier. Fair agreement was obtained between this model and exact diffusion results computed from the simulated trajectories. These results indicate that it may be possible to quantify the affect of polymeric group substitution on the transport properties of sorbed particles in similar polymers.