The reported method using differential pulse voltammetry on a platinum electrode allows selective determination of low concentrations of sulfometuron methyl (SMM, weed killer "Oust", DuPont). An electroactive species is obtained in an aqueous solution of SMM with NaClO at pH 9.0. The reaction is characterized by a yellow coloration (lambda(max) = 400 nm) with maximum absorbance measured after 30 min. The reaction product shows a cathodic peak in acidic medium, its current is linearly proportional to the SMM concentration. The potential range (E-pc around 440 mV vs. Ag/AgCl) makes the monitoring highly selective. Oxidation of the iodide ion to iodine proves the presence of a ’positive’ chlorine in SMM-Cl. Solid phase extraction of more hydrophobic SMM-Cl, when compared with SMM, from aqueous solutions allows a ten-fold increase in sensitivity of the method (ppb range or lower).