Poly(styrene-block-ethylene oxide) (PS-b-PEO) is a model hierarchically nanostructured polymer that contains PEO crystallites within microphase segregated block copolymer morphology. In order to investigate dynamics in this hierarchically nanostructured polymer, water was used as a selective, interacting probe molecule. First, the equilibrium effect of water sorption on the polymer structure was examined. Then, the coupled dynamics of structural changes and water diffusion were studied. At moderate water activity, crystallite dissolution caused water diffusion to appear non-Fickian (determined by Fourier Transform infrared - attenuated total reflectance spectroscopy). PEO deliquescence was so extreme at high water activity that dynamics could not be measured in a homopolymer control. In the block copolymer at unity water activity, a second stage of water sorption occurred that was attributed to relaxation of the glassy PS phase. Water desorption was ten times faster than sorption. Desorption is thought to be less hindered by tortuosity resulting from PEO crystallites. These results are of interest for a wide-range of applications including batteries and fuel cells, water desalination, food packaging, and biomedical implants. (C) 2016 Elsevier Ltd. All rights reserved.