Aerobic biodegradation of the 2-butyl-octyl alcohol polyethoxylate (2Bu-C8AE) was investigated under laboratory standardized conditions (Organization for Economic Cooperation and Development 301E and 310B screening test protocols) to study the effect of the 2-alkyl chain length on the biodegradation mechanism of monobranched alcohol polyethoxylates (AEs). The 2Bu-C8AE was compared with linear and monobranched AEs with short 2-alkyl chains, which were simultaneously tested under the same inoculum conditions. The metabolites identified and the primary biodegradation rate constants obtained indicate that the alkyl chain length of the 2-alkyl substituent is the factor governing the biodegradation pathway of monobranched AEs. Short 2-alkyl substituents (i.e., methyl and ethyl groups) allow the central cleavage mechanism to occur, which leads to the formation of polyethylene glycols, whereas AEs with longer alkyl substituents, such as the 2Bu-C8AE, biodegrade through hydrolytic oxidation of the alkyl and polyethoxylic chains, which leads to formation of AE metabolites with carboxylic groups on both the hydrophobic and hydrophilic moieties

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