This document establishes a valid method for separation of chemically heterogeneous polyethylene oxide (PEO) mixtures and for the determination the number and content of the chemically heterogeneous species in the overall sample. The method presented in this document serves as a technical guideline and enables laboratories to learn the principle of "critical chromatography" on a validated system. This method presented in this document with its stated system parameters is not applicable for other polymer classes, due to the diversity of the interactions between the polymer/mobile phase/stationary phase and the number of separation systems that are therefore available. The evaluation of the interlaboratory testing has shown that many error sources relate to the technique of liquid chromatography in general. Possible error sources are described in Annex A. Details on the evaluation of the interlaboratory testing are given in Annex B. Elugrams of the participants (excerpts) are given in Annex C. Investigations of the long-term stability of the test mixture are given in Annex D.