A systematic study of butazolidin and tanderil electroanalytical determination method by differential pulse voltammetry is carried out in aqueous-organic media because of their insolubility in water. Several parameters, such as solution pH, organic solvent/H2O(v/v) ratio, scan rate, and temperature have been considered, showing that tanderil oxidation voltammograms can present two peaks depending on solution pH, whereas butazolidin gives only one anodic peak at all the pH values. Both voltammograms can be used to establish electroanalytical methods of these drugs, giving a detection limit of 2.73 ppm for tanderil and 10.31 ppm for butazolidin. Moreover, in spite of their slight peak potential differences, these drugs can be determined together in their mixtures. On the other hand, our method was applied to butazolidin determination in two veterinary formulations with very good results. The proposed instrumental method was validated with another instrumental method, the Issa et al. '5 spectrophotometric one, cited in references.