TiB2 and ZrB2 have been synthesized by a mechano-chemical treatment of a mixture of titania or zirconia powder and amorphous boron followed by a relatively low temperature annealing (1100 degrees C). Both the temperature and the kinetics of the borothermic transformations are affected by the mechano-chemical treatment when the size of the particles obtained after thermal annealing of the sample milled for a short time remains in the sub-micron range. The reaction paths are different for TiB2 and ZrB2 with the formation of TiBO3 and Ti2O3 as intermediate compounds in the case of the borothermic reduction of titania, while in the zirconia/boron system a direct borothermic reduction of zirconium oxide is observed.