The occurrence of metastable long range order (LRO) along the growth direction with a periodicity twice that of a disordered alloy is now a well established result in epitaxial hcp  Co-Ru and Co-Pt alloy thin films. We first present a review of previous results obtained in Co-rich Co1−xRux alloy thin films (x = 0.25) prepared at various growth temperatures (TG) and the diffusive model developed to account for the observed behavior. This model which takes into account the competition between surface and bulk interactions and surface and bulk diffusion during the growth time reproduces the TG dependence of the LRO parameter (η) and of the lattice constants. Then we present new results on the occurrence of this LRO in Co1−xRux films for higher Ru concentrations (0.22 ≤ x ≤ 0.78) prepared at the optimal temperature for the occurrence of this LRO (570-595 K). Chemical ordering is observed over the whole explored concentration range and it is found to be in agreement with results obtained in a previous series prepared at 610 K with lower Ru content. The LRO parameter deduced from the difference in composition between two successive planes normalized with respect to the concentration of the minonty element displays a nearly symmetric dependence with concentration characterized by a maximum around x = 0.2 and a minimum around the equiatomic composition. These results are qualitatively discussed in terms of a balance between the different energetic contributions to the surface segregation that arise from the release of the elastic energy and the difference in the wetting energy of each element. The important role played by the quality of the substrate in determining the value of η is outlined.