Cesium (<5 × 10-3M) can be extracted from aqueous sodium iodide solutions into equal volumes of 0.2M I2 in nitrobenzene at 25°C with extraction coefficients as great as two thousand. Cesium may be back-extracted by equilibration with ∼4M or stronger nitric acid. The separation factor of cesium (∼10-7M) from sodium in extractions made from sodium iodide solutions into equal volumes of 0.2M I2 in nitrobenzene at 25°C remained fairly constant at ∼1500 up to ∼1M NaI. It decreased to ∼100 at 8M NaI. At this point the cesium extraction coefficient was still greater than two. The decrease in the separation factor is seen as largely due to the decrease in the ratio of the activity coefficient of the cesium ion to that of the sodium ion in the aqueous phase. Extraction data for sodium and cesium from various solutions and under a variety of conditions are given. Sodium and cesium ions are seen extracted as ion association compounds with triiodide ions. Polyiodide ions as complex as the enneaiodide are formed subsequently in the organic phase. Sodium and cesium polyiodides are thought to be essentially ionized in nitrobenzene. Extractions are believed to be the result of the “squeezing out” from aqueous solution into nitrobenzene of ion pairs of cations of small ionic potential together with large anions that are compatible with the organic solvent. The driving force behind the extractions comes largely from the reformation of the hydrogen bonded water structure in the relatively large cavities left by the extracted ion pairs.