Bilayers of a thin, nonoxidized Al film over an Ir film are prepared. The bilayers are intended to enhance the reflectance of Al below the Al plasma wavelength nm). Above the bilayers have a high reflectance comparable to that of single Al films. Ir films were deposited by electron gun evaporation on a heated substrate. Al films of various thicknesses are deposited by evaporation from a thermal source on the Ir film once it is cooled to room temperature. The bilayers are prepared under ultra-high-vacuum (UHV) conditions and their reflectance is measured in situ. Accelerated aging processes, which are performed by annealing the bilayers in UHV, show that the bilayers are stable after 1 h annealing at temperatures up to 540±25 K. The bilayers show no observable deterioration after long-term storage under normal atmosphere. The optical constants of Ir films are also obtained. In-orbit coating of optical surfaces with Ir/Al bilayers is proposed for extreme UV astronomy. © 2002 Society of Photo-Optical Instrumentation Engineers.