The objective of this study is to develop a novel in situ optical sensor for monitoring pesticides in water. The sensor consists of a polymer-coated attenuated total reflection (ATR) element or a silver halide optical fibre. The polymer film, which consists of PVC with a chloroparaffin plasticiser, acts to enrich the chlorinated pesticides and concentrates them within the penetration depth of the evanescent region of the ATR crystal or optical fibre. Measurements were performed by filling the sample cell with the aqueous pesticide solutions and recording a series of spectra over a period of time. Absorbance data were recorded for atrazine at 1577 cm(-1) and alachlor at 1104 cm(-1). Limits of detection in the region of 2 ppm have been achieved for atrazine and alachlor using a polymer-coated ATR element. Lower limits of detection can be achieved using polymer-coated optical fibres, especially with novel fibre configurations. These data establish the potential for this sensor for the use in pesticide analysis at alarm levels.