The kinetics of the reaction between the hexachloroiridate(IV) ion and iodide ion in aqueous acidic media has been monitored spectrophotometrically in the temperature range 20 to 40â °C, at ambient and at elevated pressures (up to 125 MPa). With suitable selection of reactant concentrations the reaction can be studied as simple pseudo first-order (iodide ion in excess) yielding a second-order rate constant of 1.42â à â 10³ M^{â 1} s^{â 1} at 25â °C and atmospheric pressure. This compares favourably with a value of 1.38â à â 10³ M^{â 1} s^{â 1} for the same parameter, determined previously under the same conditions. The reaction rate varies to a limited degree with variation in the conjugate base of the acids employed, which may be the result of a secondary medium effect. Potassium ions exert a significant catalytic effect, a finding that parallels those reported for other anionâ anion redox reactions. The reaction is moderately sensitive to temperature: Î Hâ ^{â ¡} ranges from 18 to 34 kJ mol^{â 1} depending on the reaction medium. All Î Sâ ^{â ¡} values are distinctly negative, suggesting a degree of molecular ordering and/or increasing electrostriction in the rate limiting step. Under all conditions employed pressure accelerates the reaction rate, resulting in large negative volumes of activation, typically â 20 to â 25 cm³ mol^{â 1}. How the thermal and activation parameters relate to the mechanism is discussed.