Abstract: A method and a device for producing a controllable and reproducible temperature gradient in a predetermined direction of heat-conducting plates for the separation of mixtures of substances in sheet-shaped separating media wherein one edge of a plate is heated by means of one or more controllable heating means and the opposite edge of the plate is cooled by one or more controllable cooling means. In this way, the energy flows through the heating means and of the cooling means are greater than the energy flow across the plate. The method and device are suitable for separating mixtures of substances wherein at least one component undergoes a thermal conversion within the temperature range of the temperature gradient. Therefore, they are suitable for the detection of and differentiation between viroids, satellite RNAs, viruses containing double-strand or circular nucleic as well as for the analysis of protein-nucleic acid complexes and for the analysis of mutations.