The paper examines how the conditions of activation preprocessing (dry and wet grinding) of B4C–Al and SiC–Al powder mixtures in a planetary-ball mill influence the formation of phases in the mixtures and the phase composition, structure, and mechanical properties of the associated hot-pressed materials. It is shown that dry grinding, unlike the conventional powder charge preparation method, induces the mechanochemical synthesis of new phases in the B4C–Al powder mixture along with its mechanical activation. The formation of new phases in the SiC–Al powder mixture does not depend on the conditions of activation preprocessing. Secondary phases form in the hot pressing of the B4C–Al and SiC–Al powder mixtures. The structure and micromechanical characteristics of the materials indicate that dry grinding of the B4C–Al and SiC–Al powder mixtures has a greater activation effect on the hot pressing process, structural uniformity, phase composition, and mechanical properties compared to the conventional powder charge preparation method.