Para-substituted phenoxide ions are characterized by a strong resonance interaction between the oxygen and the substituent when the substituent is electron-accepting. We have modeled these interactions by computing the minima of the electrostatic potential, V-min, and the average local ionization energy, (I) over bar(S,min), near the unprotonated oxygens of para-substituted phenoxide ions. The calculations of V-min and (I) over bar(S,min) were carried out at different levels of theory using both ab initio and density functional methods. V-min and (I) over bar(S,min) have been found to correlate well with the empirical substituent constant, sigma(p)(-). The calculated energy differences for the acidic dissociation of phenols and the corresponding experimental gas-phase acidities were found to agree very well. A dose linear relationship was found between V-min and the gas-phase acidities. The findings suggest that through-resonance is accounted for in our calculations and that V-min and (I) over bar(S,min) can be used as tools for predicting sigma(p)(-).