A model of discontinuous precipitation in binary polycrystalline alloys at low temperatures is presented. The proposed approach allows independent determination of the main parameters, including the interlamellar distance, the maximum velocity of the phase transformation front, and the concentration step at this boundary. This is achieved by using a set of equations for: 1) the mass transfer in the moving interphase boundary; 2) the balance of the entropy fluxes at the phase transformation front, and 3) the maximum rate of the free energy release under constraint of entropy balance. Concepts of mobility and linear interrelation between the driving force and velocity are not used explicitly. Comparison of the model calculation with the experimental results for the Pb-Sn system at different supersaturations is provided.