The influence of the Mn content on the magnetic properties and microstructure of the Fe-Mn-Co alloys has been investigated by means of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Mossbauer spectroscopy. Experiments reveal that two types of thermal-induced martensites, epsilon (h.c.p.) and alpha' (b.c.c.) martensites, form in the as-quenched alloys. epsilon and alpha' martensites coexist in the Fe-Mn-Co alloys with 13.2-17.3 wt% Mn content while only epsilon martensite appears in the case of 20.7 wt% Mn. The amount of a' martensite decreases significantly while the amount oft martensite increases with an increase in Mn content. The lattice parameters of austenite increase also with increasing Mn content. In addition, Mossbauer spectra of the alloys reveal a paramagnetic character with a singlet for the gamma (f.c.c.) austenite and e martensite phases and a ferromagnetic character with a broad sextet for a' martensite phase. The magnetic character of the Fe-Mn-Co alloys changes as Mn content increases and the ferromagnetic character disappears completely in case of 20.7 wt% Mn. (C) 2011 Elsevier B.V. All rights reserved.