Fe-C alloys were electrodeposited at high current Efficiencies hom FeSO4 solutions containing. a dicarboxylic acid with a Linear alkyl chain, represented by COOH .(CH2)(alpha). COOH (n = 0 to 7). The carbon contents of the deposits ranged from 0.1 to 3.7 weight percent (w/o), depending on the acid added to the bath. The hardness of the deposits sharply increased to a maximum value of HV 850 with increasing carbon content up to 0.6 w/o and progressively decreased at higher carbon contents. The deposits having carbon contents above 0.5 w/o exhibited a black color, because carbon-rich alloys deposited through the black oxide films in a state similar to FeO containing an organic acid or its degradation products. Fe-C alloy deposits had a body-centered tetragonal, martensite-type lattice with interstitial carbon atoms, and the axial ratios increased with increasing carbon content of the deposits. The high hardness of the Fe-C alloy deposits is mainly attributed to the solid solution hardening by interstitial carbon atoms in the martensite-type lattice. The larger grain size and FeO occluded in the bulk of the carbon-rich deposits caused the decrease in hardness with increasing carbon content.