X‐ray diffraction theory is developed for the evaluation of long range order and short range order in the alloy Cu3Au, and experiments are described whereby each type of order was measured. Values of the long range order parameter S were found to be 0.86 for 250°C, 0.81 for 370°C, and 0.79 for 380°C, the samples being held at elevated temperatures during the x‐ray exposures. At the critical temperature Tc=388°C, S suddenly dropped to zero. Above Tc short range order was found to exist, the amount present being specified by a set of parameters σi which are associated with the interatomic distances ri. The dominant short range order parameter σ1, referring to nearest neighbors, was evaluated as 0.50 for 540°C and 0.25 for 750°C.