This International Standard specifies a method, by determination of the oxygen demand in a closed respirometer, for the evaluation in aqueous medium of the ultimate biodegradability of organic compounds and waste waters at a given concentration by aerobic microorganisms. The method applies to organic compounds which a) are water-soluble under the conditions of the test; b) are poorly water-soluble under the conditions of the test, in which case special measures may be necessary to achieve good dispersion of the compound (see for example, ISO 10634); c) do not reach and react with the CO2 absorbent; d) are volatile, provided that a suitable respirometer or suitable conditions (e.g. a smaller ratio of volume head space to volume liquid medium) are used; e) are not inhibitory to the test microorganisms at the concentration chosen for the test. The presence of inhibitory effects can be determined as specified in 7.3, or by using any other method for determining the inhibitory effect of a compound on bacteria (see, for example, ISO 8192). NOTE The conditions described in this International Standard do not always correspond to the optimal conditions for allowing the maximum degree of biodegradation to occur. For alternative biodegradation methods, see ISO 15462.