We have observed a dramatic dependence of the extreme ultraviolet (EUV) reflectivity of Mo/Y multilayers on the oxygen content of yttrium. This is explained as being due to a change in the microstructure and an increase in roughness of the yttrium layers and not just to an increase in absorption owing to the amount of oxygen within the yttrium layers. We found that the best reflectivity of 38.4% was achieved with an oxygen content of 25%, which was reduced to 32.6% and 29.6% for multilayers manufactured from oxygen-free yttrium and 39%-oxygen yttrium, respectively. These results highlight the importance of including experimentally determined optical constants as well as interface roughness in multilayer calculations. In addition, the lifetime stability of Mo/Y multilayers with different capping layers was monitored for 1 year. The molybdenum- and palladium-capped samples exhibited low surface roughness and approximately 4% relative reflectivity loss in 1 year. The relative reflectivity loss of the yttrium-capped sample (yttrium with 39% oxygen) was approximately 8%. However, the reflectivity loss in all three capping layers occurred within the first 100 days after the deposition, and the reflectivity remained stable afterward.