Binary Fe-Cr alloys containing up to 25 wt % Cr carburised internally according to rapid parabolic kinetics when exposed at 900 - 1100 degreesC to gases corresponding to a(c) = 1. Internal oxidation according to slow parabolic kinetics was observed for alloys containing up to 10 wt % Cr when reacted at 900 - 1100 degreesC with po(2) = 8.7 x 10(-17) atm, and at 1000 degreesC with po(2) = 2.6 x 10(-20) atm. Precipitate compositions and volume fractions varied with the changing oxidant activity within the alloy precipitation zones, indicating the achievement of local equilibrium. Precipitate solubility products were not very small, explaining the quantitative failure of the classical Wagner model. Internal oxidation was faster than predicted by that model because low precipitate volume fractions allowed extended penetration depths. A more important factor in carburisation was the acceleration in kinetics resulting from rapid carbon diffusion along favourably oriented lamellar carbide-austenite interfaces.