The structural and magnetic properties of Co-Pt/C nanocomposite films were investigated as functions of Pt and C concentration, respectively. Under the same conditions two series multilayer samples with different Pt and C concentrations were prepared. The as-deposited films are amorphous Co-Pt-C alloys and magnetically soft. All samples were annealed in vacuum. The lattice structure is observed to transform and grain size is reduced to 7 nm. These structural transitions are accompanied by large changes in magnetic anisotropy. A maximum coercivity of approximately 5.4 kOe is obtained and a shoulder in hysteresis loops develops. These results show that Co-Pt particles are transformed by annealing from a disordered phase to ordered face-centered-cubic CoPt3 and face-centered-tetragonal CoPt, commixed in a C matrix, and the various Co-Pt crystalline particles interact. (C) 2003 Elsevier Science B.V. All rights reserved.