[en] For an effective preparation of o-iodohippuric acid-¹³¹I (Hippuran-¹³¹I), a useful medical tracer, kinetic studies were carried out for a lately reported high temperature isotope exchange labelling method. To acetate buffer (pH 5.5) or to absolute ethanol solvent a trace amount of sodium thiosulfate or trace hydrogen peroxide was added to proceed iodine isotope exchanges between o-iodohippuric acid (OIH) and radioiodide (¹²⁵I^-) or between OIH and molecular radioiodine (¹²⁵I₂), respectively at two different temperatures. Rate constants and activation parameters were measured by applying a radiopaper chromatographical separation technique. Since o-iodobenzoic acid (OIB) is known as a by-product often identified in the exchange labelling of OIH, data were also obtained for the isotope exchanges in OIB and iodide systems for comparison. The rate constant order was OIB reversible ¹²⁵I^->OIB reversible ¹²⁵I₂>OIH reversible ¹²⁵I^->OIH reversible ¹²⁵I₂, and the activation parameters for OIH were generally larger than those of OIB, (ΔH unequal sub(OIH) >ΔH unequal sub(OIB), ΔS unequal sub(OIH) >ΔS unequal sub(OIB)). Other data such as rate constants under different reaction conditions, solvent effects and activation parameters indicated that the mechanism of iodine isotope exchange between OIH and ¹²⁵I^- is predominantly nucleophilic even though electrophilic character can also be involved depending upon reaction conditions. Such a fact may well be caused by a feasible formation of hydrogen bonding type transition state due probably to the ortho substituent effect of CONHCH₂COOH