The effect of interaction between the magnetic and chemical ordering reactions on the phase equilibria between the bcc phases, A2, B2 and DO3 has been studied by using the four-sublattice Compound Energy Model (CEM) taking account of the magnetic contribution to the Gibbs energy. An interaction between the magnetic and chemical ordering arises when the Curie temperature is formulated as a function of the site fraction of elements on each sublattice. A positive interaction arises from the higher deviation of the Curie temperature of the ordered state from that of the disordered one and leads to a two-phase separation between the ferro- and paramagnetic B2 phases. A lower deviation causes a negative interaction which introduces a convexity in the Gibbs energy curve around the composition of the intersection point between the Curie temperature and the order-disorder transition temperature. As a result, two-phase separations between either para- or ferro-magnetic A2 phase and paramagnetic B2 phase are introduced.