We also show that ZnSe(L) LHs degradation slows considerably when either O2 or H2O (or both) are excluded. Exposure of ZnSe(ba) to O-enriched H2 O and 18 O2 shows that the oxygen from both of these species is incorporated into the final ZnO degradation product. These data in conjunction with DFT calculations point to a degradation mechanism whereby water and oxygen are first reduced by Se 2-in the ZnSe layer to produce Zn(OH)2 and Se 0 , followed by spontaneous conversion of Zn(OH)2 to ZnO over time.