The solubilization behavior of a poorly soluble model drug, phenytoin (PHT), under combined use of surfactants (sodium dodecyl sulfate (SDS), Tween 80) and cosolvents (dimethylacetoamide (DMA), ethanol, poly(ethylene glycol) 400 (PEG), glycerol) was examined. The solubility of PHT in the aqueous surfactant solutions increased linearly with increase of the surfactant concentration. The solubility of PHT in water–cosolvent mixtures roughly followed the log-linear model, which is widely accepted to explain the solubilization behavior of poorly soluble compounds in water–cosolvent mixtures, except for the case of glycerol, in which the solubility was minimal at 10% (w/v) of glycerol. When the cosolvents were added to the aqueous surfactant solutions, their effect on the solubility depended on the combination of the surfactant and the cosolvent. The most striking increase in solubility was observed with DMA, regardless of the type of surfactant. When ethanol was added, an increase in the solubility was observed with the Tween 80 solution, while a dramatic decrease was found with the SDS solution. The addition of glycerol or PEG to the surfactant solutions had only a minor impact on the solubility. These solubilization behaviors of PHT in the surfactant–cosolvent mixtures were partially explained by the solubility model introduced in our previous paper [Kawakami, K., Miyoshi, K., Ida, Y., 2004. Solubilization behavior of poorly soluble drugs with combined use of Gelucire 44/14 and cosolvent. J. Pharm. Sci. 93, 1471–1479]. Addition of the cosolvents to the surfactant solutions generally offered only a small advantage from the viewpoint of improving solubility because of the decrease in the solubilization capacity of the micelles.