Pitch based carbon fibers have been converted to diamond through exposure to a pure hydrogen microwave plasma. The conversion depends strongly on the process pressure. At low pressure only part of the surface of the fiber was converted to diamond, however under optimum conditions a thick continuous diamond layer could be obtained. The carbon conversion efficiency to form diamond at the high pressure was at least an order of magnitude higher than is obtained in a regular CH4+H2plasma assisted diamond deposition. ©1995 American Institute of Physics.