Ultrathin γ -Fe/Cu(001) films are unstable with respect to the fcc→bcc phase transition with the reported onset of transformation as early as at 5 ML. Here we demonstrate that with the help of a collaborative surfactant effect of carbon and oxygen, γ -Fe films up to more than 60 ML thickness can easily be produced. The interstitial incorporation of carbon atoms into the fcc lattice is the main reason for this stabilizing effect. Oxygen plays a very important role in improving the layer-by-layer growth. This strong surfactant effect of oxygen, however, reveals itself only in the presence of carbon. The interstitial carbon does not influence the magnetic properties of fcc iron significantly, except for the surface anisotropy value.