The reaction between cis and trans isomers of (Et3P) 2PtAr2 (Ar = 4-FC6H4) (1a and 1b, respectively) with I2 was studied in detail. At low temperatures, the clean formation of an intermediate was observed for each of the isomers. Upon warming, both intermediates provided the single thermodynamic product trans-(Et3P)2PtAr2(I)2 (2), where each of the iodo ligands is located trans to the aryl group in the equatorial plane. The reactions followed first-order kinetics with the intermediate formed from 1b converting into 2 at a faster rate than that from 1a. The conversion was significantly faster in toluene than in acetone, with the reaction in toluene giving a very large negative activation entropy (ca. -40 cal/mol· K -1). On the basis of the NMR analysis and kinetic data, both intermediates are proposed to be the corresponding products of the trans oxidative addition. Subsequent iodide dissociation and rearrangement of the resulting pentacoordinate Pt(IV) complex provides complex 2.