The present work describes a simple, fast and inexpensive voltammetric method for diflunisal measurement using a non-modified boron-doped diamond (BDD) electrode. The oxidation of the agent was irreversible and was a diffusion-controlled process. The sensitivity of the square wave voltammetric measurements were significantly improved when the cationic surfactant, cetyltrimethylammonium bromide (CTAB), was present in the supporting electrolyte solution. Using the square-wave mode, a linear response was obtained for diflunisal quantification in a 0.1 mol l(-1) phosphate buffer solution (pH 2.5) containing 5.10(-5) mol l(-1) CTAB at +1.07 V (vs. Ag/AgCl) (after 30 s accumulation under open-circuit conditions). Linearity was found for 0.05 to 2.0 mu g ml(-1) (2.0.10(-7) - 8.0.10(-6) mol l(-1)) with a detection limit of 0.013 mu g ml(-1) (5.2.10(-8) mol l(-1)). The developed approach could be used for the quantification of diflunisal in pharmaceutical formulations.