The characteristic crystal model is applied to the study of phase stability and thermodynamics in substitutional bcc-based Nb-Mo alloys. The basic principle and method of alloy thermodynamics which is described by the characteristic crystal model is introduced in detail. The thermodynamic properties of the Nb-Mo system are evaluated to describe the Gibbs energies of various phases including both disordered and ordered phases. The phase diagram of Nb-Mo is assessed and predicted. The results are in good agreement with experiments. An unambiguous tendency toward order with a B2 superstructure in a broad range of alloy composition is predicted.