Trioctylamine (TOA) was used as the extraction agent for reactive extraction in various active diluents. Equilibrium and hydration data for various active diluents were obtained in 0.6 M TOA systems. From these data we divided the active diluents used into three classes : active diluents containing chlorine atoms, carbon-bonded oxygen donor active diluents, and phosphorus-bonded oxygen donor active diluents. This classification was based on the characteristic groups of active diluents. The amounts of lactic acid and water in the organic phase were strongly affected by physical extraction of the active diluents. The relationships between coextracted water and lactic acid extracted were represented by slopes in drawings. The sensitivity index, which is defined as the inverse of a slope, was introduced as the measure of the water-enhanced solubilities of lactic acid. Factors affecting the reactive extraction of lactic acid and water, such as the type of active diluents, the moles of amine used, and the bulkiness of the active diluents, were explained based on the characteristic groups of the active diluents.