We investigated the effect of water-soluble alcohols (glycerol, propylene glycol, and 1-propanol) on the surfactant aggregation and its structure by phase study, surface tension measurement, small-angle X-ray scattering (SAXS) measurement, and pulsed field gradient NMR self-diffusion measurement. The phase behavior in the (water + glycerol)/C12EO8 system as a function of the glycerol content in water at constant temperature is similar to that in the water/C12EO8 system as a function of temperature, and a phase separation (clouding phenomenon) takes place at a high glycerol content. The phase separation was not observed in the other alcohol systems. However, a hexagonal liquid crystal (H-1) changes to an isotropic solution (W-m) in all the systems with increasing the alcohol content. The SAXS measurement for the H-1 and the W-m phases, and the self-diffusion measurement for the W-m phase suggest that the dehydration of the ethylene oxide chain takes place and the aggregates tend to grow with increasing glycerol content. On the other hand, propylene glycol or 1-propanol molecules tend to penetrate into the palisade layer of the aggregates and the micelles are downsized, and eventually they are broken into monomers with increasing the alcohol content. The critical micelle concentration measurement also supports the difference in the alcohol effects on the phase behavior in the aqueous nonionic surfactant system.