Abstract A surfactant mixture of didodecyldimethylammonium bromide (DDAB) and n-alkyl polyglycol ethers (CiEj) can make efficient microemulsions of decane and water. Increases in surfactant efficiency by up to a factor of four are realized as 2% C8E3 is replaced with DDAB. As little as 6% of an appropriate surfactant mixture can microemulsify equal masses of oil and water. The increase in DDAB concentration causes the spacing of the bicontinuous domains of oil and water to decrease and the correlation length of the surfactant monolayers to increase. These changes in structural parameters, as detected by small-angle neutron scattering, are in quantitative accord with theoretical calculations of changes in structure as a result of electrostatic stiffening of elastic membranes. Although the reported changes in microstructure are consistent with predictions, they alone cannot explain the observed large increases in overall surfactant efficiency.