BACKGROUND: Phenyl-urea herbicides are found in surface waters and wastewaters as a consequence of their extensive use in agriculture. Due to their pollutant power, the removal of phenyl-ureas is a priority objective in water treatment technologies. RESULTS: Four selected phenyl-ureas herbicides (linuron, diuron, chlortoluron and isoproturon), dissolved in two water matrices (a groundwater and and a reservoir water), were subjected to sequential combinations of chemical treatments and membrane filtration processes. Two specific sequences were conducted: first, a chemical oxidation stage (where UV radiation, ozone and ozone plus hydrogen peroxide were used) followed by a nanofiltration process; and second, a membrane filtration stage (using UF and NF membranes) followed by an ozonation stage. Values for the herbicide removals in the oxidation stages and for the rejection coefficients in the filtration stages are provided, and the partial contribution of the different stages is established for each combined treatment. CONCLUSIONS: High removals (over 80%) were reached for phenyl-ureas elimination by most of the combined processes tested. In the combined chemical oxidation/nanofiltration processes, the most effective was an ozonation pretreatment ([O-3](0) = 1.5 mg L-1)) followed by a NF step. In the opposite sequence filtration/chemical oxidation, the most effective was a NF pretreatment followed by the ozonation ([O-3](0) = 2 mg L-1). (C) 2009 Society of Chemical Industry