Abstract: This paper introduces a mathematical model for batch processes of crystallization according to the theory of population balance and uses Laplace transformation in its solution. On its basis a dynamic method to measure the kinetics of crystallization is developed and used for the determination and comparison of the change of kinetic parameters with different addition agents for the cooling and salting-out crystallization of NH4C1 in the production of alkali. A new method for selecting addition agent based upon dynamic data of crystallization is advanced. It is found to be easy, feasible and relatively precise. In the experiment the Coulter-counter was used to measure the required data of CSD from a large number of samples and the parameters for the kinetics of crystallization was fitted by the use of multi-variable nonlinear regression.