We have studied by angle-resolved photoelectron spectroscopy the ordered XCu₂ surface alloys formed by X=Sb and Bi on a Cu(111) substrate. We found clear analogies with the corresponding XAg₂ alloys formed by the same elements on Ag(111). The electronic states near the Fermi level are similary split by the spin-orbit (SO) interaction with the smaller splitting in XCu₂ reflecting the smaller atomic SO parameter of Cu vs Ag. The charge transfer from the Bi and Sb adatoms to the substrate atoms and their outward relaxation are different for two substrates; both determine the fermi energy of the two-dimensional electron gas.