High-temperature Knudsen cell mass spectrometry has been used to study the equilibria involving the B2N molecule over the Si-BN system. Thermal functions needed in the evaluation of the mass spectrometric equilibrium data have been calculated from available experimental and theoretical molecular parameters. The enthalpy changes for the reactions 2B(g)+Si2N(g)=B2N(g)+2Si(g), and BN(s)+B(g)=B2N(g) have been measured. The following atomization enthalpy, Delta H-a(0)o, and enthalpy of formation, Delta H-f(298.15)o, in kJ mol(-1), of 1045.5 +/- 18 and 551.3 +/- 18 for the B2N molecule have been determined from these reaction enthalpies. Atomization energies of similar molecules have been compared and discussed.