Ab initio STO-3G* MO calculations for the diphosphoric acid (H₄P₂O₇) and disulfuric acid (H₂S₂O₇) molecules reproduce the bridging bond length and angle averages observed in solid phosphates and sulfates. Potential energy curves calculated for both molecules relate the observed bridging bond length and angle variations in their solids. Orbital population analyses for both molecules suggest that all five 3d-orbitals play a role in bonding instead of just the two proposed by Cruickshank (1961). The accuracy with which the molecular geometries account for the averages and trends observed in related solids supports the assertion that the local bonding forces in solid phosphates and sulfates behave as though they are short range. / M.S.