Castor-oil-based polyurethanes (COPUs) were fabricated from 2,4-toluene diisocyanate and hexamethylene diisocyanate. Immersion weight-gain methods at different temperatures were used to measure the sorption and diffusion of acidic solutions into the polyurethane. It was evident from this study that these COPUs can indeed be used in acid conditions as they exhibited a low absorption of diffusing solutions (<1%) with the apparent activation energies of diffusion and permeation estimated to be 85 and 18 kJ/mol, respectively. Neutron activation analysis confirmed the possibility of the clustering of the acidic diffusing solutions because the acid/water molar values within the polymer matrix were significantly larger than those expected of the bulk solution. Mass spectroscopy indicated that any degradation that may have occurred may have been the result of fracture at the ester bond in the castor oil segment of the polyurethane. Tensile tests showed that the modulus of the saturated polymers remained above the values of the unsaturated ones. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 3403-3412, 2011