High-level ab initio molecular orbital theory has been used to predict the proton affinities of H2O and NH3 at the CCSD(T) level with con-elation-consistent basis sets through augmented quintuple-zeta for the former and augmented quadruple-zeta for the latter. Diffuse functions have been shown to yield faster convergence to the complete basis set limit for the prediction of highly accurate proton affinities. For these two systems, core-valence correlation effects are small, 0.13 kcal/mol, and were obtained from calculations with core-valence, correlation-consistent basis sets. The electronic component of the proton affinities are 171.56 kcal/mol for H2O and 211.97 kcal/mol for NH3. The zero-point vibrational corrections were taken from experimental values where available and from scaled theoretical values otherwise. The final proton affinity (PA) values at 298 K are PA(H2O) = 165.1 +/- 0.3 kcal/mol and PA(NH3) = 204.1 +/- 0.3 kcal/mol as compared to experimental values of PA(H2O) = 166.5 +/- 1 kcal/mol and PA(NH3) = 204 +/- 1 kcal/mol. The calculated values together with our estimated error limits suggest that the experimental value for H2O is too high by 1.5 kcal/mol.