Aim: Sodium Iodide (131 I) is a radiopharmaceutical available as oral capsule or solution, and is largely used in nuclear medicine for tyroid scintigraphy imaging and in radioiodine therapy. Radiochemical purity (% RqP) is defined as the percent of total radioactivity in the desired chemical form. The main methods to determine % RqP is paper chromatography (PC), thin-layer chromatography (TLC) and high perfomance liquid chromatography (HPLC). Some compendial methods also establish procedures for retardation factor (Rf) determination1-2. The objective of this work was to evaluate the mobile phase effect on the Rf of iodide (I-) and iodate (IO3 -) in the % RqP analysis of Sodium Iodide (131 I). Methods: Whatman 3MM PC (1.5 ?? 12.5 cm), methanol and glacial acetic acid were from Merck Millipore. Methanol and purified water in the proportion of 50%, 70%, 75% and 85% (v/v) were the mobile phases. Diluent solution containing 0.1 mg/mL KIO3, 0.2 mg/mL KI and 1.0 g/mL Na2CO3, and standard solutions of 0.2 g/mL KI and 0.4 g/mL KIO3 were prepared. In the origin position of a 10 cm paper strip, 5 ??L of the diluent solution were applied. After chromatographic separation in each mobile phase, the suitable standard solution was dripped on the iodate and iodide expected positions followed by drops of glacial acetic acid to reveal the Rf of the species with formation of a brown spot3. Results: The American and Argentine pharmacopoeial methods present iodate-131 as the main radiochemical impurity but differ in the mobile phase composition and in the color development method for iodide and/or iodate1-2. The systems advocate the use of 30 and 20 cm long stationary phases which require several hours for separation of the species, and methanol 70% and 75% (v/v), respectively. In a 10 cm strip and 70% (v/v) methanol, the Rf for iodate was 0.5-0.7 and for iodide was 0.7-0.9; with 85% (v/v) methanol, the Rf were 0.3- 0.5 for iodate and 0.7-1.0 for iodide, respectively, taking an hour and a half for the chromatografic run. The resolution obtained with 85% (v/v) methanol was better when compared to methanol 70% (v/v). Conclusion: By the use of the staining test for iodate and iodide proposed by the Argentine pharmacopoeia, it was possible to improve the % RqP analysis of Sodium Iodide (131 I), decreasing the analysis time of more than 4 hours to an hour and a half.