The structure of an equilibrium {100} domain wall between two antiphase domains of a b.c.c. binary ordered alloy of non-stoichiometric composition is investigated by minimizing the appropriate free energy in the inhomogeneous system using the Bragg-Williams and the pair approximations. Thermodynamic self-consistency requirements, similar to the Gibbs adsorption equation for interfaces, are derived and proved to hold for either approximation. The surface free energy is zero at T = 0°K, goes through a maximum as T increases and comes back to zero again at the order-disorder transition temperature. The chemical potential and the surface excess values of free energy, energy, entropy and amount of each species are given as a function of temperature and composition.