A computer model was developed to simulate the diffusion-controlled growth of planar boundaries during continuous cooling or heating using the Green's function method. Simulations of proeutectoid ferrite transformation from austenite and precipitation of cementite from both austenite and ferrite were conducted and the results were compared with calculation assuming the additivity rule which is based on the solution to the diffusion equation under isothermal condition. The additivity rule caused quite a large error in all the transformations studied. From theoretical considerations by other authors and present simulation results, it was concluded that the error occurred primarily due to the temperature variation of the (local equilibrium) solute concentration at interphase boundaries. In contrast to the usual expectation the amount of error was insensitive to the cooling rate and/or the magnitude of solute diffusivity.