Ternary Au-Cu-Pt alloys, of unequal gold and copper atomic contents and containing 2.5 wt.% Pt, are ordered isothermally at 250 or 400 degrees C after annealing at 650 degrees C and quenching in water. The alloys are characterized by transmission electron microscopy, X-ray diffraction, hardness and tensile testing. With 75 and 76.5 wt.% Au, the microstructural evolution depends on the ordering temperature. At 400 degrees C, a classical nanotwinned "polytwin" structure with, locally, two L1(0) crystal orientation variants is formed; this evolves into a two-phase, L1(0) + Al, checkerboard-like microstructure after 10(6) s. At 250 degrees C, ordering develops a stable (up to 10(5) s) structure locally containing all three L1(0) crystal variants arranged in a network of {1 1 0} twins roughly 30 nm wide. At both ordering temperatures the 78 wt.% Au alloy develops a similar three-variant nanotwinned structure that also remains stable up to 10(5) s. The 75 and 76.5 wt.% Au alloys display a peak in hardness after roughly 120 s at both 400 degrees C and 250 degrees C; with 78 wt.% Au, peak hardness is not reached at 10(5) s. With all three alloys, superior hardness and tensile strength, coupled with lower ductility, are obtained with the three-variant nanotwinned structure formed at 250 degrees C compared with the more classical polytwin structure that develops at 400 degrees C. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.