Cu–Ni/B 4 C diffusion couples were prepared by pressure-assisted sintering at different temperatures for different lengths of time under vacuum condition, and then phase composition and reaction kinetics of the products in the Cu–Ni alloy were investigated systematically. Results show that there is a reaction that occur between the Cu–Ni alloy and B 4 C, whose reaction products are Ni 2 B particles inside the Cu–Ni alloy along with a mixture of Ni 2 B and amorphous C at the interface between them. The diffusion of B into Cu–Ni alloys, important to the preparation of (Ni 2 B + C)/Cu composites, is interface-controlled and the thickness of the reaction layer containing Ni 2 B particles inside the Cu–Ni alloys can be expressed as [Formula: see text].