The equilibrium Cu–Pd phase diagram has been experimentally reinvestigated in the 300–680 °C temperature region by powder XRD involving a high-temperature technique. To reduce the annealing time needed for the system to attain the equilibrium state, nanosized specimens with a characteristic crystallite size of 5–15 nm were used initially. All previously reported phases, i.e., a disordered face-centered cubic solid solution Cu1-xPdx (A1), a one-dimensional long-period superlattice structure Cu21Pd7 (1D–LPS), intermetallic compounds Cu3Pd (L12) and CuPd (B2), except a two-dimensional long-period superlattice structure (2D–LPS), could be confirmed. Homogeneity ranges of all solid phases existing in the Cu–Pd system were determined at distinct temperatures. The largest deviation of the solubility limits was observed for the Pd-rich boundary of the Cu3Pd region (25–30 at.% Pd) and the Cu-rich boundary of the Cu1-xPdx region in the compositional range >55 at.% Pd. Based on the present experimental results, the constitutional Cu–Pd diagram was modified.