We have studied the kinetics of the disorder → order (A1 → L10) transformation in Cu–56Au nonstoichiometric alloy at a temperature of 250°C. The disordered initial state of the alloy was produced by either quenching of samples from a high temperature or plastic deformation. The results demonstrate that the rate of atomic ordering in the quenched alloy is extremely low: the transformation needs approximately two months of annealing at a temperature of 250°C to reach completion. The rate of atomic ordering in predeformed samples is even lower. In both an as-quenched and an ordered state, the lattice parameters of the alloy under investigation slightly exceed those of the equiatomic alloy. Independent of the initial state of samples, their microhardness first rises in the course of atomic ordering and then falls off. The resistivity of Cu–56Au alloy in a well-ordered state has been shown for the first time to be ρ = 7.04 × 10–8 Ω m, which is far lower than was thought previously. The data we obtained have been used to assess the ratio of the phases present (order/disorder) in different stages of annealing.