The carbon potential of a furnace atmosphere is of utmost practical importance during heat treatment of steels. The atmosphere is called active if the carbon potential differs from that of the steel and in that case there will be a transfer of carbon from the atmosphere to the steel or vice versa. This is the situation, for example, during the carburising in the case hardening process. In the other processes one rather requires an atmosphere that is inactive with respect to the steel, for example during austenitising of complex tool steels. In order to gain a deeper understanding of the behaviour of furnace atmospheres and to make possible optimisation of heat treatment operations a cooperation was started between AGA Innovation and the Royal Institute of Technology. The project involves thermodynamic calculations of the equilibrium composition and carbon potential of furnace atmospheres used in practice and the equilibrium properties of multicomponent steels.