The nature of hydrogen defect centres in II-VI semiconductors has been inferred by studying implanted positive muons, used to mimic interstitial protons. Neutral paramagnetic centres formed by electron capture fall into three distinct categories. CdS, CdSe, CdTe and ZnO show shallow-donor states, implying that hydrogen may act as an n-type dopant in these materials. ZnS, ZnSe, MgO, BeO and SrO show the more normal trapped-atom states, whilst HgO exhibits a state intermediate between the two extremes, reminiscent of the deep-donor hydrogen state in Si. Preliminary indications of shallow states in CdO and HgS and of a deep state in ZnTe remain to be clarified. In a search for a predictive model and in the light of recent theoretical notions of a common hydrogen pinning level, the systematics of the shallow-to-deep instability are correlated with the depths of the conduction-band minima.