Oxyethylated alcohol ± surfactant C12E10 ± was tested under the conditions of the die-away test with activated sludge as inoculum. The residual surfactant and its biotransformation by-products: short-chained ethoxylates (SCE) and long-and short-chained poly(ethylene glycols) (PEG) were separated and determined using the indirect tensammetric technique. A high percentage of primary biodegradation (98.5%) and high concentration of SCE and PEG was found at the end of the experiment. There is a clear sequence of biotransformation: central fission ® fragmentation of the long-chained PEG into short-chained PEG. Generally, the same pathway was observed as in the case of oxyethylated alcohol biodegradation under the conditions of the continuous flow activated sludge test.