The effects of Co and P on the discontinuous precipitation (DP) behavior of CuNiSi system alloys have been investigated using Cu4.5 mass%Ni1.1 mass%Si alloy (base alloy), Cu4.0 mass%Ni0.5 mass%Co1.1 mass%Si alloy (Co alloy) and Cu4.0 mass%Ni 0.5 mass%Co1.1 mass%Si0.05 mass%P alloy (Co + P alloy) aged at 430, 460 and 490°C. In all the alloys, slightly before attaining peak agehardening within grains, DP cells nucleated at grain boundaries and grew into the grains ahead of reaction fronts. The growth rates of DP cell for Co alloy were slower than those for Base one. Moreover, the trace addition of P to Co alloy (Co + P alloy) considerably retarded DP reaction. The kinetic analyses of DP using the Turnbull (T) and Petermann and Hornbogen (P-H) models yielded grain-boundary diffusion data. The activation energies for the base and Co alloys obtained using the T model were nearly identical and 125 and 124 kJ mol ¹1 , respectively. The values for both alloys determined from the P-H model were slightly larger than those obtained from the T model and about 130 kJ mol ¹1 . These results strongly suggested that the growth of DP cells in base and Co alloys was controlled by the boundary diffusion of Ni, and Ni or Co in Cu matrix, respectively. The smaller growth rate of DP cells in the Co alloy was ascribed to the higher number density of continuous ¤ precipitates than that of base one, which successively suppressed the boundary migration. [