S.B.Gamagedara & A.N.Navaratne Department of Chemistry, Faculty of Science, University of Peradeniya, Peradeniya, Sri Lanka. Abstract Oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene] is a selective pre and post emergence diphenyl-ether herbicide used to control certain annual broadleaf and grassy weeds in vegetables, fruit, cotton, ornamentals and rice. Here we report the use of a non electroactive stearic acid modified glassy carbon electrode as an amperometric sensor for the detection of Oxyfluorfen. Preliminary electrochemical characterization was done by cyclic voltammetry. Analytical characteristics of the sensor was evaluated by steady state amperometry. Cyclic voltammetric experiments of 0.1mM Oxyfluorfen in an ethanol water (4:6) mixed solution of 0.1M NaCl showed two reduction peaks at potentials of -0.39V and -0.80V, vs saturated calomel reference electrode. Oxidation peak was obtained at -0.28V. Scan rate dependence studies of the peak at -0.15 V vs saturated calomel electrode show that it is a diffusion control reaction. The potential of the working electrode was optimized and the optimum operational potential for the amperometric experiments was found to be -0.800 V and -0.900 V with respect to the saturated calomel electrode. The amperometric measurements at the above potentials suffered interference from the noise with the sequential addition of Oxyfluorfen at bare glassy carbon electrode. This difficulty was overcome by modifying the electrode surface with a suitable modifier. In this study non electro active stearic acid (0.5% w/v) was used as the electrode modifier. When stearic acid was deposited on the electrode surface noise levels of the amperometric results were substantially reduced. This may due to prevention of adsorption of the analyte molecules on the electrode surface and an uniform electron transfer mechanism. Slopes of calibration curves were taken as the sensitivities of the sensor. At -0.800V potential slope is 9.828x105 μAmol-1dm3 and at -0.900V potential slope is 1.1982x106 μAmol-1dm3. Signal to Noise ratio was calculated to be 7.00. The steady state amperometric response time (t-90) of the sensor was 6.3 sec. The coefficient of variation of the sensor was estimated to be 13.2%. Since the problematic noise levels can be successfully overcome by stearic acid coated glassy carbon electrode, this study demonstrates the potential utility of the sensor as an amperometric detector for Oxyfluorfen. S.B.Gamagedara, A.N.Navaratne “An Electroanalytical Sensor for detection of Oxyfluorfen (Goal)”; Sri lanka Association for the Advancement of Science,Proc.62nd Annual Sessions, Colombo,Sri Lanka, December 2006.