The influence of molecular primary and secondary structure on the transport of water has been investigated for a series of chemically similar polyimides using an infrared ATR method. The diffusivity of water generally decreased with increased chain backbone stiffness (as determined from the mechanical beta-transition), and the activation energies for diffusion generally increased with backbone stiffness. For a given backbone composition, the diffusivity was also shown to markedly increase as the density of the amorphous phase decreased. The vibrational spectra show that the absorbed water partitions into a bimodal distribution of sites, presumably representing single and clustered water molecules. This distribution changes markedly with saturation and is hence a sensitive probe of the local heterogeneity of the glassy structure.