The short-range ordered state of thermal equilibrium for a single crystal of Cu-14.4 at. %Al aged at 483 K has been investigated at room temperature by diffuse X-ray scattering. From the short-range order scattering separated by the Georgopoulos-Cohen method, effective pair interactions V 1mn were determined. At least the first 11 V 1mn were required to reproduce the topology of the short-range order scattering. A structural analysis of modeled short-range ordered crystals as well as the values for the ground state energy of various L1 2 -based superstructures suggest that at low temperatures the D0 23 structure is more stable for stoichiometric Cu 3 Al than any of the long-period L1 2 -based superstructures observed at higher temperatures.