Ab initio calculations have been performed to investigate the basis set and correlation energy dependence of computed lithium ion affinities of a series of first- and second-row neutral bases (NH3, H2O, HF, HCN, CO, PH3, H2S, HCl) and of the corresponding anions which result from the loss of H+. The basis set dependence was evaluated at fourth-order many-body Moller-Plesset perturbation theory [MBPT(4) = MP4], using the Dunning correlation-consistent polarized split-valence basis sets (cc-pVXZ, where X = D for double, T for triple, and Q for quadruple split) and these sets augmented with diffuse functions on atoms other than H and Li (aug’-cc-pVXZ). The presence of diffuse functions in the basis set lowers computed Li+ affinities and reduces the basis set superposition error. Computed aug’-cc-pVXZ Li+ affinities converge with increasing basis set size, with satisfactory convergence occurring at aug’-cc-pVTZ. Comparison with CCSD(T) results indicates that the Moller-Plesset expansion appears to be converging except for the Li+ affinities of the anionic first-row bases NH2-, OH-, and F-.