During carbonitriding of iron-manganese alloys employing liquid salt baths with high alkaline cyanate content, the presence of an unknown phase, identified as an iron-manganese carboxinitride of the (Fe, Mn) (C, N, O,) type, was observed on the sample surfaces. This phase forms easily and in considerable amounts on treating alloys containing more than 20 wt. % of manganese at temperatures of about 850 K and for times higher than 10 hours.