The kinetics of the disproportionation of hypoiodous acid to give iodine and iodate ion (5HOI reversible arrow 2I(2) + I0(3)(-) + H+ + 2H(2)O) are investigated in aqueous acetic acid-sodium acetate buffer. The rate of iodine formation is followed photometrically at -log [H+] = 3.50, 4.00, 4.50, and 5.00, mu = 0.50 M (NaClO4), and 25.0 degrees C. Both catalytic and inhibitory buffer effects are observed. The first process is proposed to be a disproportionation of iodine(I) to give HOIO and I-; the iodide then reacts with HOI to give I-2. The reactive species (acetato-O)iodine(I), CH3CO2I, is postulated to increase the rate by assisting in the formation of I2O, a steady-state species that hydrolyzes to give HOIO and I-2. Inhibition is postulated to result from the formation of the stable ion bis(acetato-O)iodate(I), (CH3CO2)(2)I-, as buffer concentration is increased. This species is observed spectrophotometrically with a UV absorption shoulder (lambda = 266 nm; epsilon = 530 M(-1) cm(-1)). The second process is proposed to be a disproportionation of HOIO to give IO3- and I-2. Above 1 M total buffer, the reaction becomes reversible with less than 90% I-2 formation. Rate and equilibrium constants are resolved and reported for the proposed mechanism.