Aqueous C-n-TAB (n = 8, 10, and 12; octyl-, decyl-, and dodecyltrimethylammonium bromide) solution and isooctane solution of H-AOT (hydrogen bis(2-ethylhexyl) sulfosuccinate) were mixed using a vortex mixer to produce a water-in-oil (W/O) microemulsion. The microemulsion was formed with the critical amount of C-n-TAB; the order of the critical amount was C-12-TAB < C-10-TAB < C-8-TAB. A possible cause for the microemulsion formation was proposed based on the screening of the electrostatic repulsion between negatively charged AOT's polar headgroups by the positively charged C-n-TAB's polar headgroups, the hydrophobic interaction between hydrocarbon chains, and hydrated water around AOT's polar headgroup.