This paper deals with new approach of measurement of the concentration profiles, concerning the redistribution of substitution elements in close vicinity of welded joint of two different steels. These concentrations have been measured in three straight lines in a plane perpendicular to the weld interface. The concentration profiles have been evaluated from experimental data by modelling of diffusion based on 2nd Fick’s law. Original computation program for nonlinear regression was evolved based on Levenberg - Marquardt`s algorithm for evaluation of diffusivities and other quantities. Three groups of measured data enable variations in methodology of their processing. If each group is processed individually then the possible differences in mutual position of concentration profiles offers the view on chemical heterogeneity of welded joint.