Mass spectrometric studies were made at high temperatures of the vapors over BeF2(s) and BeO(s) individually and of their mixture. The sublimation enthalpy, ΔH 0 ° = 55.56 ± 0.43 kcal/mol , where 1 kcal equals 4184 J, was obtained for BeF2 by the use of a log I + T versus 1/ T plot. A complex molecule appeared in the BeF2-BeO system at high temperatures which, on the basis of its m/e position, corresponded to Be2OF2. For the reaction, BeO(s) + BeF2(g) = Be2OF2(g), a second law treatment of the data gave an enthalpy value, Δ H 0 ° = 41.56 ± 1.8 kcal/mol . The data, combined with free energy functions, resulted in a mean reaction enthalpy, Δ H 0 ° = 42.6 kcal/mol . The heat of formation for Be2OF2(g), Δ H 0 ° = − 288.3 kcal/mol , was computed using the reaction enthalpy based on the second law.