AbstractThis paper discusses the biodegradability of alcohol‐based nonionics measured by the recommended legislative test procedures and how the results obtained are affected by the chemical structure of the surfactant, and thus provides guidance on the selection of materials. More detailed studies on the biodegradation of alcohol ethoxylates during the activated sludge sewage treatment process are also reported. Examination of a wide range of alcohol ethoxylates in the legislative tests shows that the majority of those nonionics of practical importance will be extensively biodegraded. Although the mathematical model used to design the treatability test is very simple and has frequently come under criticism, the predictions seem to be upheld and the results obtained appear to provide a reliable guide to what is likely to happen in practice. The sludge residence time, which has long been regarded as of particular importance by those involved in the field of sewage treatment, is clearly demonstrated to be a highly significant factor whose influence should be taken into account in any detailed laboratory study of treatability. The study of alcohol ethoxylates indicates that extensive primary biodegradation will occur even in overloaded treatment plants where sludge retention times (SRT) are likely to be short. The effect of temperature on the biodegradation is small and suggests that effective treatment will be achieved in such plants even at the lower temperatures experienced during winter. Ultimate biodegradation of alcohol ethoxylates was shown to be extensive under practical conditions and levels of “polyethylene glycol” intermediates discharged to surface waters will be low. Although alcohol ethoxylates are rapidly and extensively absorbed on activated sludge, this does not play a significant role in the removal process which is essentially one of biodegradation.