Depth profiles of ZnSe(ba) exposed to H2 O in the presence of naturally abundant O2 yield an average O/ O ratio of 8.0%, nearly 20 times higher than that detected in sample exposed to naturally abundant H2O and O2. Hence, H2O acts as a co-reactant with O2 in the degradation of these materials and not just as a catalyst. We rule out the possibility that the high O/ O signal may come from residual water at the sample surface based on consistent O/ O signal intensity during the entire 2 min etching frame (Figure ). While the roughness of drop-cast ZnSe(ba) powder films precluded quantification of sputtering depth during D-SIMS, sputtering of the more dense ZnSe suggests an upper limit to the depth profiled in our D-SIMS experiment of approximately 1 μm. To better understand the role of the ligand on LH degradation, ZnSe was exposed to isotopically enriched environments identical to those used for ZnSe(ba) degradation and analysed by D-SIMS. Exposure to isotopically enriched 18 O2 yielded a similar 4-5-fold increase in the degraded material's O/ O ratio. However, O incorporation from H2 O was ~5 times greater than that observed for ZnSe(ba). For the same degradation mechanism, the amount of O contributed from O2 and H2 O should not change between the LH and the bulk material. As several different degradation pathways are available to ZnSe on exposure to H2O and O2, we hypothesize that the surface-bound ba ligands of ZnSe(ba) restrict the number of roles H2O can play in degradation, reducing the extent of incorporation of oxygen from H2O.