The preparation of refractory metal borides of Group 4b of the Periodic Table of the Elements by reaction in the vapor phase of the corresponding metal halide, e.g., metal chloride, with a boron source reactant in the presence of hydrogen in a reactor is described. Reactants are introduced into a reactant mixing zone containing hydrogen through reactant inlet assembly means. Refractory metal boride product deposits on the exposed surfaces of the reactant inlet assembly means are reduced significantly by introducing metal halide reactant and substantially anhydrous hydrogen halide into the hydrogen stream upstream of the boron source reactant while introducing boron source reactant mixed with inert carrier gas, e.g., argon, into said hydrogen stream downstream of the metal halide and hydrogen halide streams.
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