In this study a new approach based on statistical thermodynamics is presented to calculate heterogeneous nucleation rates dependent on the surface of foreign matter present in the solution. The derivations are valid, if the number of nuclei is small compared to the number of foreign particles. In a series of crystallization experiments with the chemical system of barium sulphate, rates of heterogeneous nucleation have been determined dependent on varying chemical composition and varying amounts of foreign particles added to the solution.