The existence of complexes (clusters) in the alloy system Au-Cu was studied through determination of the copper activity for 14 different compositions at temperatures around 1100 K. The experimental data were obtained from emf measurements using stabilized ZrO2as the solid electrolyte. Using an equilibrium calculation method the relative amounts of»free»gold, of»free»copper and of complexes were indicated. The best fit to experimental data was obtained with a model assuming the existence of the complexes Cu3Au. CuAu, CuAu3and CuAu9.