Fractionation factors have been calculated for the H/D isotopic equilibrium reactions of O2H5+ clusters. The calculations used ab initio geometries and force constants to evaluate the reduced isotopic partition functions for each isotopic species. We find good agreement with experimental gas-phase measurements [Graul, S, T.; Brickhouse, M. D.; Squires, R. R, J. Am. Chem. Sec. 1990, 112, 631-639] at higher levels of Hartree-Fock theory. By contrast, Moller-Plesset second order perturbation theory (MP2) yielded somewhat poorer agreement with experiment, presumably because the calculated hydrogen bond distances are underestimated at this level.