The present work is concerned with a thermochemical study of phthalazine, quinazoline, quinoxaline carried out by combustion calorimetry of small amount of substance (a few milligrams) using a CRMT rocking calorimeter equipped with a microbomb.This study enables us to determine the enthalpy of combustion and of formation of the three compounds in the condensed state. Using the previously obtained values of sublimation enthalpy [2], we could calculate the values of their enthalpy of formation in the gaseous phase. All these experimental values enable us to:discuss the relative stability of the three molecules,propose energy contributions for C-N and N-N bonds.