Fe-Co-C alloy coatings were electrodeposited from a sulfate solution containing citric acid and L-ascorbic acid as additives. Crystal structure and mechanical property of the coatings were examined by using X-ray diffraction (XRD) and Vickers hardness tester. The atomic ratio of Fe/Co in Fe-Co-C alloy coatings is essentially the same as that in carbon-free Fe-Co alloy coatings. However, the crystal structure of Fe-Co-C alloy is quite different from that of Fe-Co alloy. In the case of Fe-Co-C alloys, coatings with 30.0 to 83.8 mass % Co, an intermediate phase was detected. XRD analysis indicated that the phase had the same crystal structure as the cubic cobalt phase that was produced by the evaporation of Co on cold substrate under He atmosphere. However, the cubic structure changes to a tetragonal system by the incorporation of carbon. This intermediate phase is quite unstable and then transforms to bcc-Fe and hcp-Co phases during pulverization process. The hardness of Fe-Co alloy coatings was improved by the codeposition of carbon. The maximum hardness of ca. Hv 800 was obtained for the Fe-10.9 mass % Co-1.1 mass % C alloy. The improvement in hardness of Fe-Co-C alloy coatings is discussed in detail from the viewpoint of alloy composition and crystal structure. (c) 2005 The Electrochemical Society. All rights reserved.