The electrochemical oxidation of three carboxylic acids (formic, oxalic, and maleic) using diamond thin-film electrodes was studied using voltametric and galvanostatic electrolysis techniques. The voltametric study shows an anodic current peak that indicates the existence of a direct oxidation reaction at the electrode surface. Galvanostatic electrolysis studies confirm the existence of mediated oxidation reactions and indicate almost total mineralization of the waste with a virtually direct transformation of carboxylic acids into carbon dioxide (no intermediates were detected) regardless of the current intensity, initial organic acid concentration, temperature, and supporting media. Experimental results obtained in the galvanostatic electrolysis study can be fitted satisfactorily (CV < 10%) using a simple model that considers both kinetic and mass-transfer processes.