The BSO (Bansal, Shah, O’Connell) equation (J. Colloid Interface Sci. 75, 462, 1980) was derived as an empirical condition for maximum water solubilization in microemulsions stabilized by anionic surfactants, in relation to the cosurfactant (alcohol) and oil chain lengths. The present study examines, for the first time, the applicability of this equation for nonionic microemulsions. By constructing pseudo-ternary phase diagrams and measuring the area of the single phase regions as well as the water contents at the boundary, we find that the solubilization capacity is directly related to the alcohol chain length only if the alcohol solubility in both the aqueous and oleic phases is limited. In addition, we try to integrate the concept of chain length compatibility with the competition between the natural curvature and the critical radius of the interface. The attempt to unify the experimental results may serve as a guideline for optimization of microemulsion systems.