A theoretical analysis is presented concerning the study of the influence of the substituent X on the acidity of a set of parasubstituted phenols X-C6H4OH. Linear correlation analyses between calculated Gibbs free energy changes on dissociation of parasubstituted phenols in the gas phase (DeltaGdegrees), the hydrogen atom charge Of X-C6H4OH (q(H)), and the electronegativity of the fragment X-C6H4O (chi(XC6H4O)) show that acidity is related to q(H) and chi(XC6H4O). When the electronegativity increases, the hydrogen atom charge and the acidity of the parasubstituted phenols increase as well. These results suggest that there is no direct interaction between the X substituent and the hydrogen atom; the effect is primarily due to the interaction of X with the C6H4O group. The substituent does transmit its effect through the 7 system of the benzene ring, and q(H) shows the effect of the electronegativity of the fragment X-C6H4O on the hydrogen atom. Analytical equations obtained from density functional theory (DFT) and the hard and soft acids and bases principle (HSAB) provide guidelines to obtain a quantitative description of the fit parameters in the linear correlation equations. The results obtained suggest that qH is representative of the gas-phase acidity of parasubstituted phenols.