Water vapor transport properties for the polymers Kapton HReference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Atomic Energy Commission to the exclusion of others that may be suitable. and Parylene C were determined over a temperature range of 20 to 55°C. Activation energies and entropies for permeation as well as partial molar free energies, heats, and entropies of dilution were calculated for water vapor concentrations ranging from 3 × 10−6 to 1 × 10−3 mole H2O per cm3 of polymer. Mylar A was tested to extend the available data for partial molar heats and entropies of dilution and to compare permeation and diffusion results with the corresponding values in the literature. Diffusion coefficients were measured using the time‐lag technique of Barrer but employing a modified test apparatus. Equilibrium sorption isotherms at 30°C were obtained for Mylar A and Kapton H with a Cahn microbalance. The ratios of the permeability to diffusion coefficients as measured from time‐lag experiments agreed with solubility coefficients within 3% for Mylar A and within 12% for Kapton H. Both polymers obeyed Henry's law. The results were interpreted in light of polymer polarity and morphology.