The critical thickness for pseudomorphic Co growth on Cu(001) is found to be independent of the onset of lattice constant relaxation. The pseudomorphic film relieves strain by local formation of orthomorphic growth patches within the pseudomorphic matrix. This unusual relaxation mechanism of electrodeposited films is in contrast to current belief of film relaxation. Moreover, a tetragonal distortion of the fee Co unit cell in the orthomorphic growth regime indicates residual strain in films of up to at least 100 monolayers thickness. (C) 2000 Elsevier Science B.V. All rights reserved.