As part of an extensive study devoted to the development of new high-Al steels, a CALPHAD-type assessment of the Al–Fe–C system has been carried out. In place of the usual cementite, these steels show precipitation of the so-called κ carbide that is an ordered form of austenite. Inconsistencies between the scarce experimental information in the Al–Fe–C system and extrapolations from the available Al–Fe and Al–C descriptions made it necessary to revise them. This has been done in part by using new ab initio calculations as well as phase diagram data. The κ carbide has been described as an ordered form of the fcc phase, with its Gibbs energy calculated as the sum of a disordered and an ordered contribution. The additional parameters needed to express the ordered part were evaluated using phase equilibria information and new ab initio data related to the κ carbide. With this approach, addition of new alloying elements, like Mn or Ni, for extrapolation to higher-order systems should be straightforward.