The dry environment consisted of a sealed desiccator filled with enough 3 Å molecular sieve to adsorb sufficient water from the desiccator. The humid environment consisted of a sealed desiccator containing saturated aqueous Mg(NO3)2 (ca. 30 mL) to form a closed environment of 50% R.H. at room temperature. To form a O2-free closed environment of 50% R.H., saturated aqueous Mg(NO3)2 (ca. 30 mL) was degassed by three freeze-pump-thaw cycles. The degassed solution was placed in a desiccator sealed within a N2 atmosphere glovebox. For H2 O/H2 O and 18 O2/ 16 O2 isotope experiments, both ZnSe(ba) and ZnSe were placed in a 22 mL sealed container containing either 1 mL H2O with ambient atmosphere, 1 mL H2 O with otherwise ambient atmosphere, or H2O (natural isotopic abundance) with 18 O2. ZnSe(ba) and ZnSe were exposed to these environments for 10 days at ambient temperature, after which samples were subject to reduced pressure (~0.01 Torr) for no fewer than 2 days to remove adsorbed vapor or gas prior to analysis by D-SIMS.