The acid-catalysed hydrolysis of isopentenyl phosphate appears to proceed via a reversible hydration–dehydration mechanism in which the initially formed 3-hydroxy-3-methylbutyl dihydrogen phosphate is partially converted into the acid-labile dimethylallyl dihydrogen phosphate. The allylic phosphate and pyrophosphate esters have been found to be hydrolysed by a unimolecular process with C–O bond fission, whereas the non-allylic pyrophosphates undergo hydrolysis by cleavage of the central P–O–P linkage.