Using the results of a thermodynamic analysis, a Co-Cr-Mo alloy was successfully nitrided in nitrogen at temperatures of 1073-1473 K. The near-surface microstructure of the treated Co-Cr-Mo alloy was characterized using X-ray diffraction, field-emission scanning electron microscopy, electron probe microanalyzer, and transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy. The results indicated that the highest nitriding efficiency was achieved at the treatment temperature of 1273 K, with the size and coverage of the nitride particles on sample's surface increasing with an increase in the treatment duration. After nitriding at 1273 K for 2 h, numerous nitride particles, consisting of an outer Cr2N layer and an inner pi phase layer, were formed on top of the nitrogen-containing gamma phase, and some pi phase also precipitated in the alloy matrix at the sub-surface level. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.