This work investigated the speciation and solubility of indium (In) and its flotation separation from aqueous solution. Lowest theoretical In solubility was predicted for pH 5.5−8.5, and the solubility increased as it became more acidic or more alkaline. Experimentally, 20 mg/L of In completely dissolved at pH < 3.0, which agreed with theoretical prediction. At pH 3.6−9.9, In showed slightly higher solubility than predictions. There was good agreement at pH > 9.9. Flotation reaction was described by pseudo-first-order kinetic model and its reaction rate increased with increasing nitrogen flow rate. The results indicated that pH played a key for the flotation process. As pH increased from 4.0 to 6.8, removal efficiency rose from 65.0% to 95.8% when applying surfactant SDS as collector. The efficiency decreased to 56.7% at pH 9.9. Lower removal efficiency was found once using CTAB. Electrostatic interaction between collector with both soluble and colloidal In was the main mechanism for In flotation.