Polyurethane (PU) is an emerging resin matrix for fiber reinforced polymer (FRP) composites. In the present article, effects of immersion in water or alkaline solution on water uptake and thermomechanical properties of a PU were investigated. A traditional epoxy resin matrix was investigated for comparison. As found, unlike the epoxy (hydrolysis occurred in case of alkaline solution), water and alkaline solution immersion showed a similar effect for the PU resin. For both immersion, the water uptake of the PU system reached the equilibrium content of 2.5%-2.7%, in accordance with the Fick's law. Glass transition temperature (T-g) of the PU decreased rapidly after the first month of immersion and then leveled off. Both immersion for a short term (e.g., 1 month of immersion) led to an obvious decrease in the tensile strength, while long-term immersion brought in obvious recovery. Remarkable increase in the elongation at break (up to 16.6%) due to immersion was found. The higher the immersion temperatures, the less the degradation of the tensile strength. The variations of the thermal-mechanical properties were attributed to the water-induced plasticization and the postcuring-induced increase of the crosslinkage. Compared with the common epoxy system, the PU system showed better resistance to the immersion. POLYM. ENG. SCI., 58:2276-2287, 2018. (c) 2018 Society of Plastics Engineers