Theoretical computations were carried out to determine the structure and molecular parameters of the BNSi molecule. The most stable isomer is found to have a BNSi linear geometry. Thermal functions as derived from the theoretical computed molecular parameters were used in the evaluation of the thermodynamic properties of BNSi from high-temperature Knudsen effusion mass spectrometric equilibrium data. From the reactions analyzed by the second-law and third-law methods, the enthalpy of formation, Delta(f)H(0)(o), and of atomization, Delta(a)H(0)(o), in kJ mol(-1), for BNSi, were obtained as 398 +/- 16 and 1078 +/- 17, respectively.