The heats of formation of BHmCln have been computed at the G-2 level of theory, which is estimated to be accurate to +/-2 kcal/mol. The calculated ideal gas values in kcal/mol are Delta H-f298(0)(BH) = 105.8, Delta H-f298(0)(BCl) = 42.5, Delta H-f298(0)(BH2) = 79.4, Delta H-f298(0)(BHCl) = 34.5, Delta H-f298(0)(BCl2) = -6.8, Delta H-f298(0)(BH3) = 24.9, Delta H-f298(0)(BH2Cl) = -19.0, Delta H-f298(0)(BHC12) = -60.7, Delta H-f298(0)(BCl3) = -98.6.