In order to develop an effective separation process for tin (IV) from indium (III) to produce high purity indium free from tin contamination, basic investigations on solvent extraction of indium (UI) from sulfuric acid solution containing chloride ion added for the purpose of suppressing precipitate of tin hydroxide were conducted at 30 degrees C employing Cyanex 925 and trioctylphosphine oxide (TOPO) in toluene as the solvents. Cyanex 925 quantitatively and selectively extracted tin (IV) from indium (III) in the low concentration region of sulfuric acid. From the concentration dependencies of the reactant species, i.e., the reagents, hydrogen ion and chloride ion, on the distribution ratio, it was elucidated that indium (III) is extracted as the solvated complex of the type, H.InCl4.2Reagent. Co-extracting hydrogen ion with both of these extraction reagents and the equilibrium constants for the extraction reactions were evaluated. The value for Cyanex 925 was half of that for TOPO, which was attributed to the sterically hindered structure of alkyl radicals of the former reagent.