A novel modular, mid-infrared, evanescent wave fibre optic sensor for the detection of hydrocarbon pollutants in water has been designed, constructed and tested. The key sensor design feature, its modularity, utilises simple, low cost, optical components and provides the potential for low cost production of rugged field based sensors. In addition, novel optical design upgrades, such as tapered, coiled and bevelled fibres, can be incorporated to improve detection limits. The current set-up uses a broadband source with backreflecting optics coupled to a fibre optic sensing element, coated with an analyte-enriching polymer that concentrates the analyte in the sensing region. The fibres output end is filtered at an analyte absorption peak and coupled to a thermopile detector whose signal is read into a computer for analysis. Results show the system's use as a pollutant detector, with benzene quantified down to 500 ppm using a PVC polymer coating. Response times for a range of benzene concentrations are sufficient for real time quantification. Other analytes can be quantified using an appropriate enriching polymer and bandpass filter centred on an absorption peak for the analyte in question or an analyte family. Ongoing component developments allow for the evolution of the sensor system's specifications. (c) 2005 Elsevier B.V. All rights reserved.