To better understand the roles of O2 and H2O in II-VI LH degradation, we used D-SIMS to track the source of oxygen in degraded LHs. ZnSe(ba), which degrades over relatively short time scales, was exposed to environments having isotopically enriched H2 O or 18 O2. Three separate samples of ZnSe(ba) were exposed to environments of 50% RH having no isotopically enriched compounds, H2 O as the source of water, or O2 as the source of oxygen, respectively. These samples were left at ambient temperature for 10 days in their respective environments. The samples were then subjected to reduced pressure (~0.01 Torr) to remove adsorbed species (e.g., H2 O or 18 O2) prior to characterization with D-SIMS.