Isatin derivatives possess a broad pharmacological profile and clinical applications, and the electrochemical behaviour of isatin-3-hydrazone, 7-methylisatin-3-hydrazone, isatin-3-semicarbazone, isatin-3-thiosemicarbazone, 5-bromoisatin-3-thiosemicarbazone and 5-nitroisatin, at a glassy carbon electrode, using cyclic, square-wave and differential pulse voltammetry over a wide pH range, was investigated and compared with isatin. The oxidation mechanism of isatin-hydrazone and isatin-semicarbazone derivatives is an irreversible, pH-dependent, adsorption-controlled process, and occurs in two consecutive charge transfer reactions with the formation of electroactive oxidation products, and their reduction mechanism is similar to the reduction of isatin, one single irreversible, pH-dependent cathodic process. The redox processes of isatin-hydrazone derivatives are associated with the hydrazone group, and for isatin-semicarbazone derivatives with the semicarbazone group. The oxidation mechanism of nitroisatin is also similar to isatin but the reduction is an irreversible, pH-dependent process that occurs in two consecutive charge transfer reactions with the formation of one electroactive reduction product. The results showed that all functional groups attached to the isatin ring structure give rise to different redox mechanisms, and the isatin derivatives investigated present new redox properties. (C) 2012 Elsevier B.V. All rights reserved.