The Co70Mo30 alloy structure formed at different temperatures was studied by means of transmission electron microscopy and photoelectron spectroscopy. It has been found that at temperatures below the solidus temperature the alloy has a tendency toward phase separation which leads to formation of a quasi-cellular microstructure in the form of clusters strongly enriched in Mo and Co, respectively, but existing within a common fcc lattice of the solvent. At low temperatures, an allotropic phase transition alpha-Co -> epsilon-Co occurs in the alloy and the strain contrast, arising at such transitions is superimposed on the quasi-cellular structure. The Co70Mo30 alloy acquires high hardness due to the fact that the crystal lattice in the clusters strongly enriched in Mo remains face-centered. (C) 2012 Elsevier BM. All rights reserved.