This part of ISO 5815 specifies a determination of the biochemical oxygen demand of waters by dilution and seeding with suppression of nitrification. This part of ISO 5815 is applicable to all waters having biochemical oxygen demands greater than or equal to 3 mg/l of oxygen (the limit of determination) and not exceeding 6 000 mg/l of oxygen. For biochemical oxygen demands greater than 6 000 mg/l of oxygen, the method is still applicable, but the errors caused by the necessary dilutions can influence the analytical quality of the test method and the results are to be interpreted with circumspection. The results obtained are the product of a combination of biochemical and chemical reactions. They do not have the rigorous and unambiguous character of those resulting from, for example, a single, well-defined, chemical process. Nevertheless, they provide an indication from which the quality of waters can be estimated. The test can be influenced by the presence of various substances. Those which are toxic to microorganisms, for example bactericides, toxic metals or free chlorine, will inhibit biochemical oxidation. The presence of algae or nitrifying microorganisms can produce artificially high results. Annex A describes alternative incubation periods. Annex B describes multitesting, which can be used to obtain enhanced precision or to demonstrate the presence of substances toxic to microorganisms. Annex C provides precision data.