This document specifies the determination of the biochemical oxygen demand of waters by dilution and seeding with suppression of nitrification after 5 d incubation time. It is applicable to all waters having biochemical oxygen demands usually between 3 mg/l and 6 000 mg/l. It applies particularly to waste waters. 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. Then the results are to be interpreted with circumspection. The results obtained are the product of a combination of biochemical and chemical reactions with participation by living matter which behaves only with occasional reproducibility. 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. NOTE A limit of detection, as clearly defined in statistics, cannot be determined for this method. The lower limit of the working range results from the limit conditions which are explained in 10.1 ("consumption rule") and in Table 1. Annex A describes the method for an incubation time of 7 d. Annex B describes multitesting, which can be used to obtain enhanced precision or to demonstrate the presence of substances toxic to microorganisms. Annex C describes a seeding method in which the seeding is performed directly when applying the analysis approach. Annex D provides performance data.