The purpose of this study was to estimate the screening of electrostatic repulsions between the polar headgroups of AOT(-) by alkali metal counterions and to explore the relationships between the screening effect and the phase behavior of H-AOT-based W/O microemulsions. The screening effect was evaluated by means of critical micelle concentration (CIVIC) data using the pyrene 1:3 ratio method with aqueous Solutions containing M-AOT (where M+ = Li+, Na+, K+, Rb+ and Cs+) to form normal micelles, and by counterion binding constants, determined from plots of CMC versus counterion concentration. The order of the screening effect was found to be K+ approximate to Rb+ > Cs+ > Na+ > Li+. Interestingly, the order does not follow the hydration size dependence of the alkali metal counterions. An aqueous MOH Solution containing a given concentration/H-AOT/isooctane was emulsified at a water content (w(0) = [water]/[H-AOT]) of 10 to produce H-AOT-based W/O microemulsions. The phase behavior and size variation were investigated by FT-IR and DLS measurements. The emulsified mixture separates into two phases at lower MOH concentration due to all insufficient screening effect. When the concentration is increased to a level Sufficient to intensify the screening effect, W/O microemulsions are formed without phase separation at lower KOH and RbOH concentrations compared to CsOH. A period of standing after the emulsification and a higher concentration of NaOH compared to KOH, RbOH, and CsOH are required to form W/O microemulsions. W/O microemulsions are not formed ill the case of LiOH. These results indicate that the formation of a W/O microemulsion with H-AOT is strongly correlated with the order of the screening effect. A possible cause for the difference in the screening effect is proposed based oil hydration of the polar headgroups and counterions, as evidenced by FT-IR spectral data, i.e., symmetrical sulfonate stretching and O-H stretching.