The phenomenon of ordering is discussed, in the context of thermodynamics, before introducing the crystallographic aspects of typical ordering reactions in iron alloys. The respective magnetic specific heats of ferromagnetic cobalt and nickel are much smaller at 80% and 40% that of iron below the Curie temperature, and 50% and 25% above the Curie temperature, emphasising the predominant role of ferromagnetism in iron. The fact that the Debye temperatures and electronic specific heat coefficients of ferrite and austenite are equal implies that the expansion coefficients of these phases should also be equal. The configurational Gibbs free energy is given by assuming that the enthalpy of ordering is equal to the internal energy, and adding the contribution from the change in configurational entropy. The quasichemical solution model has a better treatment of configurational entropy which accounts for a non-random distribution of atoms. The entropy is a thermodynamic function of state and it is additive.