This International Standard specifies a method, by analysis of biochemical oxygen demand, for the evaluation in an aqueous medium of the "ultimate" biodegradability of organic compounds at a given concentration by aerobic microorganisms. The conditions described do not necessarily always correspond to the optimal conditions for allowing the maximum value of biodegradation to occur. The method applies to all organic compounds which are sufficiently water soluble to prepare a stock solution or poorly water soluble when using special dosing techniques. Due to the low concentrations of test compound at the beginning of the test, normally no special precautions for the toxicity of the test compound to the microorganisms of the inoculum is necessary; if required a parallel inhibition test can be performed.