The current work addresses a thermochemical study regarding the compounds N-methylphenothiazine and N-methylphenoxazine. The excellent agreement between the experimental and computational gas-phase enthalpy of formation values obtained for the N-methylphenothiazine reinforced the validation/ calibration of the computational methodology established, allowing the use of it for the homologous oxygen derivative. The computational studies were also extended to the attainment of gas-phase molar heat capacities at different temperatures, dipole moment, electrostatic potential energy maps mapped onto electron density isosurface, and frontier orbitals of N-methylphenothiazine and N-methylphenoxazine. The experimental techniques used were the Knudsen mass-loss effusion, Calvet microcalorimetry and combustion calorimetry aiming, respectively, the determination of the temperature-vapour pressures dependences, the enthalpy of sublimation and the massic energy of combustion of N-methylphenothiazine. These quantities were used to derive the corresponding enthalpy of formation in the gas phase, at T = 298.15 K, (271.3 +/- 4.1) kJ . mol(-1). The results obtained for the enthalpies of formation are discussed and compared with related compounds, providing an opportunity to evaluate the effects in the enthalpies of formation associated with the substitution of the hydrogen of the amino group by a methyl group. (C) 2015 Elsevier Ltd. All rights reserved.