Effects of Co additions on the formation of Fe16N2 have been investigated by observing nitrides formed on thin-film surfaces. Thin films of Fe-Co alloys with a (100) surface sputter-deposited on MgO(100) substrates are exposed to a mixed gas of NH3 and H2. The 16:2 nitride was observed to form on a surface of Fe-10 at. % Co film at a nitriding temperature of 500 °C. The formation temperature is 50 °C higher than for pure iron. The amount of the formed 16:2 nitride has been found to be 3× larger for Fe-10 at. % Co than for pure iron.