A c(2 x 2) structure formed by adsorption of Mg atoms on Cu(0 0 1) at room temperature was determined by a tensor low-energy electron diffraction (LEED) analysis and a first-principles total-energy calculation. Both studies conclude that in the c(2 x 2) structure every second Cu atom in the first layer is substituted by Mg. Structural parameters obtained by the LEED analysis and the first-principles calculation are in good agreement and the interlayer distance between Mg and the first Cu layer is determined to be 0.55 and 0.60 Angstrom, respectively. The interlayer distance between the first and second Cu layers is contracted by about 5% from the bulk value. The calculation shows that the total adsorption energy at substitutional sites is 0.26 eV per Mg atom larger than that at hollow sites. The reason for large stability of the substitutional c(2 x 2) structure is examined and discussed theoretically. (C) 2000 Elsevier Science B.V. All rights reserved.