Polyurethanes are frequently used in underwater applications. They are required to resist the marine environment over long periods of immersion but the multiplicity of resin formulations makes material selection difficult. Accelerated tests are essential to guarantee long term integrity. This paper presents results from an experimental study in which polyurethane samples of two hardnesses, 40 and 90 Shore A, have been subjected to immersion in artificial sea water for periods up to two years at temperatures from 50 to 100 degrees C. In parallel samples have been immersed at sea in the Brest Estuary for up to five years. Mechanical properties have been measured on tensile specimens after ageing. The accelerated test results and FTIR analysis indicate that hydrolysis can occur. However, an estimation based on a linear Arrhenius extrapolation indicates that the timescale for 50% property loss at sea temperatures is in excess of 100 years. The results from sea ageing confirm that these materials retain 100% of their initial tensile properties after five years of immersion. (c) 2007 Elsevier Ltd. All rights reserved.