Computational studies were performed in an effort to understand the relative reactivity of oxoesters and thioesters in nucleophilic acyl transfer reactions. Transition state models were developed for the reactions of methyl acetate and methyl thioacetate with hydroxide, ammonia, and methylcyanoacetate carbanion. Quantum mechanical calculations based on these models reproduced experimental observations that oxoesters and thioesters have similar reactivity toward hydroxide while thioesters are about 100-fold and at least 2000-fold more reactive than oxoesters toward amine and carbanion nucleophiles, respectively. NBO analysis was performed to elucidate the role of electron delocalization in reactant and transition state stabilization. These calculations indicate similar losses of delocalization energy for the oxoester and thioester in going from the reactants to the transition state in reaction with hydroxide while the loss of delocalization energy is significantly greater for the oxoester in reactions with the viler nucleophiles. Bond rotational analysis of the transition states for the reactions with hydroxide and ammonia provide support for an important role of the P-X --> sigma*(C-Nu) interaction (X = O or S of the oxoester or thioester respectively, Nu = nucleophile) in governing the reactivity of oxoesters and thioesters in nucleophilic acyl substitution.