A method is proposed for on-line UV spectroscopy and data analysis are applied to determine nitrate and nitrite nitrogen (NO(x)-N) in various waste water matrices. Comparison with standard analytical method was performed. Sensor calibration with pure substance reference materials delivered sufficient results for characteristic municipal waste water matrix, but for waste water with substantual industrial contribution and for landfill leachate matrix match calibration was required to enhance trueness. The described method concept is well suited for on-line monitoring. (C) 2010 Elsevier Ltd. All rights reserved.