In investigating the order‐disorder transformation in CuPt, cell parameters were determined from specimens quenched at intervals from 25°C to 890°C. The rhombohedral deformation of the ordered, layered structure was found to diminish with increasing temperature, disappearing at a critical temperature of 815°C. Integrated intensity measurements of superstructure reflections from quenched powder and briquette samples determined that the long range order,S, decreases slowly with increasing temperature to a value of 0.77 atTc, where it changes abruptly to zero, with the transformation belowTcbeing homogeneous. A simple theory is developed relating the rhombohedral deformation toS.Cold working an ordered sample was found to transform the alloy almost completely from the ordered rhombohedral cell to the disordered cubic cell. From measurements of diffuse intensities diffracted by briquettes aboveTc, Fourier transform methods determined parameters which fixed the local order as being local layering configurations, in contrast to the usualA‐likes‐Btype of order.