Thermodynamic calculations have been performed to establish the partial pressure relationships between the gaseous species BO and CO at an isotherm during the reduction-nitridation of boric anhydride. This relationship enables us to establish the condition for the formation of boron nitride ceramic phase by selecting a suitable nitriding atmosphere. Relevant Gibbs free-energy equations have also been considered for establishing the phase equilibrium relationships over a wide range of temperature in the B-C-N-O system. The importance of BO gas formation and its subsequent reduction to BN is also discussed.