The equilibrium constants of formation of the gas phase species Ar-N2 (T-shape and linear isomer, their equilibrium mixture) are evaluated from recently computed quantum-chemical data. It is shown that the isomeric relative stabilities can be interchanged so that the isomers can exhibit comparable concentrations in some temperature regions. The computed overall equilibrium constant is combined with the observed temperature and pressure profiles in Titan’s atmosphere and the altitude profile of the heterodimer mole fraction in equilibrium with N2 and Ar is estimated. The mole fraction decreases with altitude and close to Titan’s surface is found to be 8 x 10(-2)%.