It was found that the lower peak hardness and faster overaging of a Co‐containing Cu‐Be alloy quenched from a low solution treating temperature is associated with a large amount of grain boundary precipitation. A similarly large amount of grain boundary precipitation, coupled with low peak hardness value, is obtained if the alloy contains a small amount of Cr impurity, even if the quenching temperature is normal. The grain boundary reaction resulting from the Cr impurity can be eliminated by using high solution treating temperatures.