This paper presents polarographic (dc and differential pulse) and voltammetric (linear-sweep cyclic voltammetric) studies of the electroreduction of 2-chloroisonicotinic acid (2-chloro-4-pyridinecarboxylic acid), 2-CISO, at mercury electrodes. The dissociation constants of 2-CISO were obtained from the UV-visible absorption spectra (pK of 0.5 +/- 0.03 corresponding to the -COOH group) and by potentiometric titration (pK of 3.46 corresponding to the protonation-dissociation of the heterocyclic nitrogen). The electrochemical studies were performed in the acidity range 2.7 M H2SO4 to pH 7. Above the last pH value no signals were observed. The overall reduction process involves the uptake of four electrons. Kinetics parameters such as Tafel slopes and electrochemical reaction orders have been determined at potentials corresponding to the foot of the waves. From the results a reaction mechanism can be proposed, consisting of an electrochemical-chemical process having two reversible electron transfers preceding a chemical step. At the scan rates used in cyclic voltammetry, the rate-determining step is the second irreversible electron transfer. At pH>0.5 the recombination of the carboxylate anion with the H+ ion takes place prior to the reduction process. (C) 2008 The Electrochemical Society.