This International Standard specifies a method for the evaluation of the “ready” and “ultimate” biodegradability of organic compounds at a given range of concentrations by aerobic microorganisms. In this context, this International Standard also gives specific definitions for the terms “ready” and “ultimate”. The method applies to organic compounds which are: a) soluble at the concentration used under the conditions of the test [dissolved organic carbon (DOC) concentrations of 10 mg/l to 40 mg/l]; b) non-volatile or having a negligible vapour pressure under the conditions of the test; c) not significantly adsorbable on glass and activated sludge; d) not inhibitory to the test microorganisms at the concentration chosen for the test. The method is not suitable for waste waters, as they usually contain significant amounts of water-insoluble organic carbon, which is not included in DOC measurements.