15N isotope effects in the nitro group and 18O isotope effects in the phenolic oxygen have been measured for the hydrolysis of ethyl p-nitrophenyl phosphate catalyzed by several metal ions. Co(III)–cyclen at pH 7, 50 °C, gave an 15N isotope effect of 0.12% and an 18O one of 2.23%, showing that P–O cleavage is rate limiting and the bond is ∼50% broken in the transition state. The active catalyst is a dimer and the substrate is presumably coordinated to the open site of one Co(III), and is attacked by hydroxide coordinated to the other Co(III). Co(III)–tacn under the same conditions shows a similar 15N isotope effect (0.13%), but a smaller 18O one (0.8%). Zn(II)–cyclen at pH 8.5, 80 °C, gave an 15N isotope effect of 0.05% and an 18O one of 0.95%, suggesting an earlier transition state. The catalyst in this case is monomeric, and thus the substrate is coordinated to one position and attacked by a cis-coordinated hydroxide. Eu(III) at pH 6.5, 50 °C, shows a very large 15N isotope effect of 0.34% and a 1.6% 18O isotope effect. The large 15N isotope effect argues for a late transition state or Eu(III) interaction with the nitro group, and was also seen in Eu(III)-catalyzed hydrolysis of p-nitrophenyl phosphate [Bioorg. Chem. 28 (2000) 283].