The surface core-level shift has been measured for Cu atoms at the (100) surface of a single crystal of a Ni-5% Cu alloy. Surface-sensitive high-resolution measurements of AlK_alpha excited photoemission from the Cu 2p_3/2 core level show surface-shifted features at a reduced binding energy 0.42 eV below the bulk binding energy. The magnitude of this shift is interpreted in terms of a quasichemical approach. The relative magnitudes of the surface and bulk components of the 2p_3/2 peak measured by low-angle electron emission exhibit a dependence upon the emission azimuth related to the surface crystal directions of the (100) plane. A simple atomic shadowing or electron blocking model is proposed to account for this. Peak-shape and peak-broadening factors are discussed for both the surface and bulk components.