The electrochemical reduction of isonicotinic acid NRCOOH and its protonated from H+NRCOOH has been examined in an aqueous medium between Hc, = 0 and pH 6.7. As shown by cyclic voltammetry and polarography, a global 2e-, 2H+ reversible transfer is followed by two successive first-order chemical reactions and a 2e-, 2H+ reduction (ECiCzE process). A mechanism is proposed, beginning with the 2e-, 2H+ reduction of YRCOOH (where Y is N or H+N) to the formal diionized forms YRC-(OH)(OH: ), where the first chemical reaction Ci is an internal proton transfer which yields the hydrate YRCH(OH)z; this reaction, which is analogous to the internal proton transfer taking place in the case of nitronic acids in acidic medium has been shown previously to occur in the case of methyl isonicotinate. The rate constant of this process is 2 x 102 s-i in neutral medium and 2 x 104 s-i in acidic medium. The second chemical reaction Ca (dehydration), which is much slower (rate constant kd < 1.6 x 10m3 s-l), involves the loss of a Hz0 molecule to give 4_formylpyridine, which is itself easier to reduce than isonicotinic acid. The variations of the total current and the dehydration constant with pH are analyzed in detail, and compared to those observed for Cformylpyridine.