Iridium terminated silicon spacer layers on Mo/Si multilayer mirrors fabricated by dc magnetron sputtering are investigated for temporal stability. Samples maintained in atmosphere for periods of more than 3000 hours show: Ir terminating layers > 16 angstrom thick are stable, layers with thicknesses between similar to 10 and 16 angstrom are not stable, and thicknesses between -7 and 10 A are stable after a loss m reflectance of similar to 1%. Extreme ultraviolet (EUV) reflectance. atomic force microscopy. sputter Auger electron spectroscopy, and x-ray diffraction measurements indicate that the terminating layer is, in reality, an alloy with graded composition that is Ir rich at the surface. The compositional gradient causes a degradation in reflectance from 66% for Si terminated multilayers to similar to 63% for Ir terminated samples (thickness 18.7 angstrom). A sudden onset of oxidation induced silicon transport for deposited layers of Ir similar to 10-16 angstrom thick produces a degradation in EUV reflectance upon exposure to atmosphere, Accelerated lifetime testing of Si and Ir (18.7 angstrom) terminated multilayer mirrors show a Delta R/R reflectance loss of 0.2% for the Ir terminated sample and similar to 3% for the Sit terminated sample, Further development on Ir terminated multilayer stacks to increase reflectance is needed. (c) 2005 American Vacuum Society.