Sodium di(n-octyl) phosphinate was synthesized, purified, and used as a surfactant to form reverse micelles in isooctane at (22 +/- 1)degrees C by the contact method, which yields a Winsor II system. Two cosurfactants were investigated: decanol and di(n-octyl) phosphinic acid, and the ionic strength of the aqueous phase was adjusted with sodium chloride. For both cosurfactants, a minimum mole ratio of cosurfactant to surfactant was required to form reverse micelles and solubilize an appreciable amount of water in the organic phase. The maximum water uptake was obtained at this minimum mole ratio. No water uptake was obtained above a maximum value of the mole ratio of cosurfactant to surfactant, The maximum mole ratio was about 1 for di(n-octyl) phosphinic acid independent of acid concentration. For n-decanol, the maximum mole ratio decreased from about 3 to about 1 as the alcohol concentration increased from 0.15 to 0.30 m. The maximum water uptake increased with increasing cosurfactant concentration.