A modification of the system of determining equations for the direct-transformation phenomenon in shape-memory alloys is proposed. It differs from the known micromechanical model of [1-3] that in the new system the rate of change of the phase deformation is orthogonal to the surface on which the phase transition begins in stress space. It is shown that the new system is uniquely solvable for the rates of change of the stresses. The uniqueness theorem is proven for solutions of corresponding boundary-value problems in the mechanics of deformable solids for shape-memory alloys.