Thin B-C-N and B-N films containing various proportions of cubic boron nitride (c-BN) have been obtained in the present study by plasma assisted chemical vapour deposition (PACVD) as a result of the in-situ decomposition of either dimethylamine borane (CH3)(2)NH-BH3 or borane-ammonia BH3-NH3 in the hydrogen plasma. The structure and composition of the films are strongly affected by the precursor to hydrogen ratio in the gas phase. The films deposited at high values of the hydrogen excess contain c-BN and are free of sp(2)-hybridized boron nitride. The mechanism of depositing sp(3)-hybridized boron nitride is proposed, which includes the selective etching of sp(2)-hybridized BN and the formation of metastable BNHx species in the hydrogen plasma.