A Raman study on hydrogen donors in ZnO reveals the properties of bond-centered hydrogen (H BC ) and hydrogen trapped in the oxygen vacancy (H O ). The donors are identified by their electronic 1s → 2s(2p) transitions and their characteristic local vibrational modes. The H O donor was detected preferentially below the sample surface, where a high oxygen vacancy concentration is generated by thermal treatment of the samples. H BC and H O exhibit different thermal stabilities and their concentrations depend strongly on the sample history. Molecular hydrogen is an essential part of the interplay of the two hydrogen donors.