<p>The gas phase acidities of phenol C₆H₅OH, benzoic acid C₆H₅COOH and <i>o</i>-, <i>m</i>-, and <i>p</i>-hydroxybenzoic acids HOC₆H₄COOH were calculated at the MP2/6-31+G(d) and CAM-B3LYP/6-31+G(d,p) levels of theory and the values were closed with the experimental <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\Delta_{acid} G^{0}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="normal">Δ</mi> <mrow> <mi mathvariant="italic">acid</mi> </mrow> </msub> <msup> <mi>G</mi> <mn>0</mn> </msup> </mrow> </math></EquationSource> </InlineEquation> values reported in the literature. The resonance effect operating in the acidity properties of the molecules were studied by analyzing the electronic density in terms of the Fukui function of their anionic compounds C₆H₅O(−), C₆H₅COO(−), <i>o</i>-, <i>m</i>-, and <i>p</i>-(−)OC₆H₄COOH and <i>o</i>-, <i>m</i>-, and <i>p</i>-OC₆H₄COO(−). The Fukui function <i>f</i> ⁻(<b>r</b>) for electrophilic attack explained the effect and suggested how the resonance contribution describe the anions stability.</p> Graphical abstract <p>Analysis of charge delocalization in the phenoxide and carboxylate anions of <i>p</i>-hydroxybenzoic acid using the Fukui function for electrophilic attack</p>