There are several carbides in the binary iron-carbon (Fe-C) system that is richer in carbon than cementite. χ-carbide is of particular importance amongst the iron carbides as a catalyst in the Fischer-Tropsch process in which hydrocarbon products are synthesised from mixtures of carbon-monoxide and hydrogen. Unfortunately, the discrepancies are sufficiently large to make the relative stabilities of the carbides difficult to assess with confidence. These calculations usually are representative of 0K and zero pressure whereas the formation energies in practice are sensitive to temperature. During carburisation reactions where the activity of carbon is sufficiently large, Hagg carbide precipitates in preference to cementite. ε-Carbide in steel, given time, definitely transitions into cementite; the evidence supports this over a large temperature range. Given that ε-carbide is less stable than cementite, and that the principal deformations needed to generate e are larger than for cementite, it is not clear why ε-carbide is a precursor to cementite.