The composite G3(MP2)//B3LYP approach was considered for the calculation of the gas-phase standard molar enthalpies of formation, at T = 298.15 K, of 2-, 3-, and 4-formylphenols and of N-, 2-, 3- and 4-formylanilines. The calculated results for the 2- and 4-formylphenols are in excellent agreement with the experimental results available in the literature with differences lower than 5 kJ/mol, supporting the computational strategy considered. Based on available experimental data for 2-formylphenol, a new value (Δ(f)H(m)o(g) = −241.2 ± 2.8 kJ/mol) is suggested. The same approach was used in the calculation of enthalpies of isomerization of the formylphenols to several compounds, namely, benzoic acid, 1,3-benzodioxole, phenylformate, 2-furanacrolein, and tropolone and also in the calculation of C−H, N−H, and O−H bond dissociation enthalpies, enthalpies of deprotonation of the C−H, N−H, and O−H bonds, and ionization enthalpies. It was found that the C−H bond is the easiest to cleave in all compounds. Finally, the introduction of the formyl substituent in phenol and aniline is found to decrease the calculated enthalpies of deprotonation in the parent compounds, and the ionization enthalpies have values between those calculated for phenol or aniline and benzaldehyde. The calculated data for these properties are in very good agreement with experimental results.