X-ray reflectivity and spectroscopy, voltammetry, and scanning probe microscopy were used to investigate the dealloying of CuxAu (with x ≈ 4) below the critical potential. We studied ultrathin films deposited on oxidized silicon wafers with a thickness of 2.2, 8.5, and 43 nm, which had been determined precisely by X-ray reflectivity. With the help of quantitative X-ray fluorescence (XRF) analysis, the average composition of the films before and after the leaching was determined. In this way, the amount of material going into solution could precisely be analyzed. A compositional depth profile was obtained by angle-dependent hard X-ray photoelectron spectroscopy before and after the dealloying. Two novel findings are reported. (1) Close to the Cu/Cu2+ redox potential, a pronounced increase in current is observed in the voltammogram. While this charge transfer seems to indicate the dissolution of Cu2+, XRF analysis proves that no copper has been lost to the solution. (2) The XRF analysis shows that some gold...