The STO-3G optimized structures of three series of carbon acids, CHnX4−n, wheren = 1, 2, or 3 and X = F, CN, or NO2and their corresponding conjugate bases are compared with the limited number of experimentally determined structures. The 6-31G* equilibrium geometries ofare included as a check on the reliability of the STO-3G structures of the anions. Although the effects of successive substitutions on the STO-3G structures are generally systematic, a few apparent anomalies are readily explained in terms of resonance stabilization. Calculations at the 3-21G level, on the STO-3G optimized structures, indicate that the stabilization energy associated with increasing the number of orbitals in the basis set is approximately additive. The stabilization energies are 0.865 ± 0.007 au per fluorine substituent, 0.661 ± 0.008 au per cyano substituent, and 1.561 ± 0.027 au per nitro substituent.