The structures of complexes between amides and metal ions were examined by FAB mass spectrometry and collision-induced dissociation (CID). Zn 2؉ was coordinated by the amide carbonyl oxygen atom of N-tetradecylacetamide (1). In contrast, Ca 2؉ and Mg 2؉ were coordinated by the amide group of 1 in both the keto and enol forms. The catalytic role of Zn 2؉ at the active site of the hydrolases might partly be explained by the effective attack of Zn 2؉ on carbonyl oxygen atom of the scissile amide group.