1. The mechanism by which refractory metals and boron react with one another during vacuum boriding was studied and the optimum impregnation conditions and thicknesses of the resulting boride phases were determined, ensuring the best combination of density, adhesion to the base, and various other properties of boride coatings. It is shown that the process of boron atom supply to a surface being impregnated is effected through solid, vapor, and gaseous constituents of charge materials. 2. The growth kinetics and phase composition of reaction products were investigated and the reactive diffusion parameters of boron in refractory metals were calculated. It was established that a correlation exists between the reactive diffusion parameters of boron in refractory metals and the statistical weight of atoms with stable d5 configurations. 3. The relationships were determined between the thicknesses of diffusion layers and the heats of formation of the main phases in boride coatings, and also between the ratio of heats of formation and the ratio of phase thicknesses in diffusion boride layers.