Endurance testing of Ru-capped multilayer mirrors (MLMs) at the NIST synchrotron facility has revealed that the damage resulting from EUV irradiation in a water-dominated environment is nonlinear and may be influenced by competing oxidation and carbon-deposition processes. Concurrent results from two different environmental chambers reveal non-intuitive relationships between reflectivity loss and the admitted water-vapor partial pressure, the ambient background-gas composition, the presence or absence of hot filaments in the chamber, the EUV intensity and the irradiation dose. We discuss possible mechanisms and propose further experiments to test them. Determining the MLM lifetime from accelerated tests is a very difficult task. It is crucial that any lifetime testing procedure involves duplicate exposures for consistency, and, if possible, testing in multiple facilities.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.