A thorough investigation of the Cu-Au system has been taken by means of X-ray diffraction studies. Alloys were annealed at appropriate temperatures for different periods (one month, three months, six months and one year) and slowly cooled down to room temperature, or quenched at 300° and 600℃, with the purpose to elucidate the phases and the phase transformations that might occur. Lattice parameters were accurately determined in order to study their variations with compositions and heattreatments. The stacking periods of the ordered structures with long periods were investigated, particularly in their relations to compositions and temperatures, and by means of quenching alloys after having been annealed for various periods of time, the ordering process near the equiatomic composition was studied. Under the conditions of heattreatment mentioned above, the entire system consists of six different phases: α1 is the primary solid solution of Au in Cu, α′1 the superstructure corresponding to Cu3Au, α2 the primary solid solution of Cu in Au, α′2 the superstructure corresponding to CuAu3, k the superstructure corresponding to CuAu I, and k′ the superstructure corresponding to CuAu II. The most noteworthy is the fact that with the increase of the period of annealing, the ordered regions extend gradually, while the two-phase regions gradually narrow down, and in the slowly cooled alloys after one year's treatment, the two-phase regions almost disappear. This leads the authors to conceive that the two-phase coexistence in the Cu-Au system is in a state of metastable equilibrium. As for the α′2 phase, the most clear superlattice lines appear not at the stoichiometric composition but at 68 at.% Au. The k′ phases which appear at both sides of the equiatomic composition are partially transformed into k phases after the alloys being treated for one year. By the equiatomic composition, both the maximum transformation temperatures of the k phase and the k′ phase are not exactly at the equiatomic composition but at 49 at.% Au or even less.The measurement of lattice spacings shows that: the relation between the mean lattice spacings of the fundamental unit cells and the compositions is a continuous curve indicating positive deviation from Vegard's law. In the a and a′ regions, a increases with the Au content. In the k′ regions, for alloys where the Au contents are less than that in the equiatomic composition, a increases with the Au content while c decreases, and meanwhile c/a deviates gradually from 1 ; and for alloys where the Au contents are more than that in the equiatomic composition, a decreases gradually with the increase of the Au content, while c increases abruptly, and c/a gradually approaches 1. At the compositions where k′ transforms into k or vice versa, both a and c undergo abrupt changes.The influence of the period of annealing to lattice spacings has been fully considered. The lattice spacings in those parts of the phase regions wherein the phases do not change with the heattreatment are practically unchanged within the experimental conditions cited above. In the α′2 region, the transformation from disorder to order causes sharp drop in the lattice spacing. In the k regions, for alloys where the Au contents are less than that in the equiatomic composition, a increases with the period of annealing; and for alloys where the Au contents are more than that in the equiatomic composition, both a and c decrease with the increase of the period of annealing. But in all k regions, the volume of the fundamental unit cell decreases with the increase of the period of annealing. The authors are therefore of the opinion that the volume of the fundamental unit cell should be taken as a general measure of the degree of order.The law governing the indices of the superlattice lines present in the k′ structures has been discussed in details as well as their correspondence to those present in the k structure. The stacking periods in the k′ structures have been accurately determined by measuring the distances between the doublets arising from the splitting of the k superlattice lines as k is transformed into k′. The variation of stacking periods with compositions is continuous. The farther the alloy is from the ideal composition, the longer is the stacking period. And for the same alloy, the higher the temperature, the longer the stacking period. The stacking period may be odd, and may be a non-integer.In the metastable two-phase regions, not only the lattice parameters, but also the stacking periods of the k′ phase vary with compositions. The two-phase coexistence is composed really of two structural forms with the same composition. Purely from ther-modynamic considerations, it has been shown that the order-disorder transformations in the Cu-Au system are transformations of the second order.