[en] In this contribution we investigate the chemical and thermodynamic parameters of the isotopic exchange reaction between radioiodine as iodide ion (¹²⁵I^-) and iodo hippuric acid isomers (i.e. O^-, m^- and p-iodo hippuric acid). The reaction was performed on molten ammonium acetate (m.p. 114 degree C) as exchange medium for 5-10 min. by allowing no-carrier added (n.c.a) radioiodine (¹²⁵I^-) to react with O^-, m^-, and p-iodo hippuric acid at 120 degree C. The absolute radiochemical yield varied as a function of initial inactive iodo hippuric acid used. Also, the different parameters affecting the radiochemical yield of the isotopic exchange reaction were investigated. The isotopic exchange reaction revealed that radioiodine as ¹²⁵I^- for iodine-127 of o^- and p-iodo hippuric acid isomers exchange occurs more rapidly than iodine-127 of m-iodohippuric acid isomer exchange. The results indicate that the reaction occurs by nucleophilic substitution as second order reaction. Sine, we were also interested in iodo hippuric acid labelling with short lived radioiodine, we tried to optimize the labelling followed by determination of the radiochemical and radiopharmaceutical purity by chromatographic analysis.The ultimate purpose of our use of ¹²⁵ I^- was to centralize the different chemical reaction conditions