A self-consistent set of thermochemical data for 33 molecules in the B-N-Cl-H system are obtained from a combination of ab initio electronic structure calculations and empirical corrections. Calculations were performed for both stable and radical species. Good agreement is found between the calculations and experimental heats of formation for most molecules containing B, H, and Cl. In addition, the BAC-MP4 and experimental heats of formation for H3B:NH3 are also in reasonable agreement, suggesting that the bond additivity parameters chosen for B-N bonds will provide reasonably accurate heats of formation for compounds containing this type of bond. Transition-state energies for two reactions involving BCl3 and NH3 are also predicted. Polynomial fits of the predicted thermodynamic data over the 300-4000 K temperature range are included in the Supporting Information.