The electrical resistivity and Vickers hardness of quenched and aged high-purity copper-titanium -aluminum alloys have been measured in order to study influences of aluminum addition on the electrical conductivity and hardness of copper-titanium alloys. The electrical resistivity of the quenched Cu-Ti-Al alloys showed that aluminum addition slightly increases the solid solubility limit in Cu-Ti alloys. The results of the present investigations samples showed that the hardness increases and the electrical resistivity decreases with aging, and the aluminum addition slightly decreases the precipitation rate. This result is considered to correspond to an increase in the solid solubility limit. In addition, the contribution of the aluminum addition to solution hardening is very small, although the precipitation hardening is observed in Cu-Ti-Al alloys.