Phenmedipham is an herbicide used especially in the sugar beet harvest to fight against broad-leaved weeds and studies of its voltammetric behavior and detection have not been done before. The superior properties of carbon nanotubes such as electrical conductivity, mechanical strength, and wide potential ranges make them attractive for chemical sensors and the phenmedipham compound gave an oxidation peak at +1320 mV (vs. Ag/AgCl) on the nanostructured multiwalled carbon nanotube paste electrode. Square wave voltammetric measurements recorded for phenmedipham showed that the peak current increased linearly between 0.02 and 2.0 mg/L with a regression coefficient of r = 0.9989, and the limit of detection and limit of quantification were calculated as 6.96 mu g/L and 23.2 mu g/L, respectively. The applicability and the selectivity of the improved square wave voltammetric method for the phenmedipham assay were investigated in the presence of certain herbicides and fungicides such as carbendazim, benomyl, aclonifen, ethofumesate, metamitron, and p-acetamsidide (methacetin) Phenmedipham at 1 mg/L with the same amount of these pesticides was determined to have recoveries of 103.5 +/- 0.07, 94.5 +/- 2.8, 104.3 +/- 1.8, 101.9 +/- 3.1, 93.8 +/- 1.7, and 101.3 +/- 1.8, respectively (n = 3). The method was also applied to the phenmedipham assay in saturated tea sugar prepared as a spiked natural sample and 1.0 g/L phenmedipham in the sugar solution was successfully determined with a relative error of -5.0% and a relative standard deviation of 3.16%.