The thermochemical properties of Norharmane, Harmane, and Harmine were investigated using DSC, combustion calorimetry, thermogravimetry, and G3B3 computational methods. DSC measurements enabled accurate determination of melting temperatures and fusion enthalpies. Complementary IR, NMR, and HPLC analyses performed for Harmine indicate that partial degradation occurs during the melting process, becoming more evident at higher temperatures (above ~330 ◦C). The standard enthalpies of formation in the solid state were 159.6 kJ·mol⁻¹ (Norharmane), 80.5 kJ·mol⁻¹ (Harmane), and −47.0 kJ·mol⁻¹ (Harmine). Using sublimation enthalpies derived from TGA, the gas-phase formation enthalpies were established as 282.7, 186.0, and 87.4 kJ·mol⁻¹, respectively. Homodesmotic G3B3 calculations showed excellent agreement with experimental data, with absolute deviations below 1.5 kJ·mol⁻¹. The combined results reveal a consistent thermodynamic stability trend in both phases: Harmine > Harmane > Norharmane.