The formation of titanium, zirconium and hafnium diborides by reaction between metal dioxides and elemental boron in vacuum in the temperature range 1000 °–1750 °C has been investigated. It has been established that titanium diboride with a composition close to that of stoichiometric TiB 2 is obtained after a heattreatment of the reactants at 1700 °C for I h; HfB 2 is obtained after heat treatment at 1750 °C for 2 h. Experimental results show that higher temperatures are necessary for the preparation of stoichiometric ZrB 2 by the borothermic method. The structure of HfB 2, and its stability at boiling point and at room temperature in the presence of acids, mixtures of acids, mixtures of acids with oxidizing agents, and alkaline solutions has been studied.