The lattice parameter of expanded austenite was determined as a function of the content of interstitially dissolved carbon in homogeneous, carburized thin stainless steel foils. For the first time this expansion of the face-centered cubic lattice is determined on unstrained austenite. It is found that a linear relation holds between the lattice parameter, a, and the occupancy of the octahedral interstices, yC. The results are compared to other values in the literature and discussed in terms of stressed and unstressed austenite.