Described is a process for the separation of a sample mixture for analytical reason based on two-dimensional gel electrophoresis. The method is involving a first separation in a first gel strip on the basis of isoelectric points and a second separation in a second gel on the basis of molecular size. When starting the separation in the second dimension the buffer solution for transferring the compounds separated in the first dimension into the second dimension gel is containing sodium dodecyl-sulfate (SDS) and by applying an electric field the SDS migrates electrokinetically into the first gel strip, and the compounds are being complexed simultaneously with SDS.