AbstractBranched alkylbenzene sulphonates, recalcitrant xenobiotic compounds, were treated aerobically in a single‐tank activated‐sludge reactor and also by means of three continuous‐stirred‐tank reactors (CSTR) in series. For the single‐tank reactor it was observed that, the longer the hydraulic retention time, the greater were the reductions in biological oxygen demand (BOD), chemical oxygen demand (COD) and methylene blue active substances (MBAS). Reductions of 80%, 60% and 63% were achieved in the levels of BOD, COD and MBAS, respectively. In the case of the three‐CSTR system, acclimatization of the microorganisms was achieved 15 days after start‐up. For this system, the reductions in the levels of BOD, COD and MBAS were 80%, 68% and 69%, respectively.