Composites made up of boron, carbon, and nitrogen were studied using a combination of chemical vapor deposition (CVD) and high pressure (HP) techniques. The CVD/HP process comprised deposition of C-x(BN)(1-x) composites where x = 0.03 or 0.8, followed by heat treatment of the deposited samples under pressures from 6 to 10 GPa using an octahedral-anvil apparatus. Also a sample prepared by nitriding of boron and carbon to compose x = 0.5 was studied up to 16 GPa. A diamond-type solid solution was obtained after the C-0.8(BN)(0.2) sample was treated at 10 GPa and 1600 degreesC or at 8 GPa and 1700 degreesC whereas the C-0.03(BN)(0.97) and C-0.5(BN)(0.5) samples exhibited decomposition into carbon and BN under all the P, T conditions studied. Sintered compacts consisting primarily of the cubic BN and carrying hardness with a maximum value of 54 GPa were obtained from C-0.03(BN)(0.97).