A method for determining the movement of particles, particularly impurities, in a medium, under the influence of a changing interface between two neighboring phases. In a first step, the temporal and/or local evolution of said interface is determined. In a second step, the movement of said particles in dependence of the temporal and/or local evolution of the phase interface as determined in the first step is calculated. Optionally, the distribution of the particles within the medium at a certain time is then determined.