A sequential model of the decomposition of the bcc FeCr binary alloy is formulated that takes into account the configurational and magnetic contributions to the free energy. Using the results of ab initio calculations, the theory of regular solutions is generalized by considering the contributions of the magnetic entropy, the concentration dependence of the exchange interactions, and the mixing energies. The resulting expression for the free energy makes it possible to construct the boundaries of the two-phase region of the bcc FeCr alloy in good agreement with the experimental data, as well as to predict the position of the spinodal, below which the formation of highly dispersed states should be expected when starting from a homogeneous state.