The reactions between boron and phosphorus to form the boron phosphides are described. The elements were compacted into pellets and dropped into a preheated reaction zone. The reaction between zinc phosphide and boron also resulted in the formation of the boron phosphides and the complete removal of zinc in the vapor phase. Cubic BP formed initially and decomposed into the rhombohedral B13P2above 1130°C. The formation of B13P2by decomposition of BP was rapid and complete at 1600°C. Some properties of BP and B13P2 are summarized. The B13P2formed by decomposition was a very fine powder and large crystals were grown in metallic melt