Poly(monochloro-p-xylylene) (Parylene C) coatings are commonly used in a number of applications due to their robust properties and unique ability to be deposited directly from the vapor phase. Recently, Parylene C has been used in new medical devices, where an accurate assessment and understanding of liquid transport and its relationship to the polymer structure is critical in design and evaluation. In this study, the diffusion of liquid water in Parylene C coatings was examined as a function of the thermal history with in situ time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy, while the polymer structure was investigated with in situ X-ray scattering. Anomalous transport behavior was observed, where dynamic infrared spectra provide evidence of water diffusion and water-induced polymer relaxation occurring on similar time scales. Both of these phenomena were quantified and regressed to a diffusion relaxation model to determine the diffusion coefficient and polymer relaxation time constant. After thermal treatment of the Parylene C coating, the water diffusivity reduced by 2-fold, which can be attributed to the increase in crystallinity and the evolution of a new crystalline phase in Parylene C. (C) 2011 Elsevier Ltd. All rights reserved.