The electrochemical reduction of isonicotinic acid NRCOOH and its protonated from H(+)NRCOOH has been examined in an aqueous medium between H-0 = 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 (EC(1)C(2)E 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)(OH2+), where the first chemical reaction C-1 is an internal proton transfer which yields the hydrate YRCH(OH)(2); 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 10(2) s(-1) in neutral medium and 2 x 10(4) s(-1) in acidic medium. The second chemical reaction C-2 (dehydration), which is much slower (rate constant k(d) < 1.6 x 10(-3) s(-1)), involves the loss of a H2O 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 4-formylpyridine.