The use of 17O-NMR to investigate bond cleavage during the hydrolysis of sulphate esters in water enriched in 17O is described. Despite the inherent disadvantages of 17O for NMR studies, this work shows that, in favourable cases, 17O-NMR of 17O-enriched species is a powerful and sensitive tool for mechanistic studies. It is particularly useful when OS cleavage occurs, resulting in the formation of S 17O 16O 3 2− (5% 17O), which can easily be detected at the biologically relevant μmole level. The method complements those using H 2 18O and has the advantage that in principle 17O can be detected in either of the hydrolysis products with little or no purification. It has been shown that sulphatase A (aryl-sulphate sulphohydrolase, EC 3.1.6.1) cleaves the OS bond while functioning as a cerebroside sulphatase, as it does when functioning as an aryl- or glycosulphatase.