Phosphorothioate esters are sometimes used as surrogates for phosphate ester substrates in studies of enzymatic phosphoryl transfer reactions. To gain better understanding of the comparative inherent chemistry of the two types of esters, we have measured equilibrium and kinetic isotope effects for several phosphorothioate esters of p-nitrophenol (pNPPT) and compared the results with data from phosphate esters. The primary 18O isotope effect at the phenolic group (18kbridge), the secondary nitrogen-15 isotope effect (15k) in the nitro group, and (for the monoester and diester) the secondary oxygen-18 isotope effect (18knonbridge) in the phosphoryl oxygens were measured. The equilibrium isotope effect (EIE) 18knonbridge for the deprotonation of the monoanion of pNPPT is 1.015 ± 0.002, very similar to values previously reported for phosphate monoesters. The EIEs for complexation of Zn2+ and Cd2+ with the dianion pNPPT2- were both unity. The mechanism of the aqueous hydrolysis of the monoanion and dianion of pNPPT, the diester ethyl pNPPT, and the triester dimethyl pNPPT was probed using heavy atom kinetic isotope effects. The results were compared with the data reported for analogous phosphate monoester, diester, and triester reactions. The results suggest that leaving group bond fission in the transition state of reactions of the monoester pNPPT is more advanced than for its phosphate counterpart pNPP, while alkaline hydrolysis of the phosphorothioate diester and triester exhibits somewhat less advanced bond fission than that of their phosphate ester counterparts.