Seven alloys of the ternary Au-Cu-Pt system, containing 75 or 76.5 wt.% Au with 0-15 wt.% Pt and heat-treated to a fully ordered condition, are examined using in-situ synchrotron radiation to determine transformation temperatures and phases formed, and to investigate mechanisms of ordering and disordering. These experiments are complemented by differential scanning calorimetry. Diffraction data show that the alloys can be separated into three groups according to their stable low-temperature ordered phase(s), namely L1(0), L1(2), or the two combined. Platinum increases transformation temperatures in comparison with binary AuCu, two-phase ordered alloys showing the highest transformation temperatures. Details of the evolution of the X-ray diffraction peak structures upon heating and cooling point to significant differences between mechanisms of disordering and ordering: whereas ordering visibly proceeds at significant undercooling by nucleation and growth, disordering appears, from the visible shifts in peak position, to progress in more homogeneous fashion within the alloy. (C) 2011 Elsevier Ltd. All rights reserved.