The phase behavior and thermodynamic properties of IL/O microemulsions (ionic liquid dispersed in oil) containing 1-dodecyl-3-methylimidazolium bromide ([C(12)mim]Br) + pentan-1-ol + octane + 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and W/O microemulsions (water dispersed in oil) containing [C(12)mim]Br + pentan-1-ol + octane + water were studied and compared. The area of the single-phase domain in [bmim][BF4] microemulsion systems is larger than that in water-containing systems. Both the mole fractions of the n-alkyl alcohol in the oil phase X: and the mole fractions of the n-alkyl alcohol at the interfacial layer X, in IL/O microemulsion systems are always larger than X-a(o) and X, in W/O systems, respectively. IL/O systems have smaller absolute values of the free enthalpy values -Delta G(o -> i)(o) than those in W/O systems. In the IL/O microemulsion systems, a larger number of cosurfactants at the interfacial layer facilitates the formation of a smaller droplet IL/O microemulsion. The effects of n-alkyl alcohols, alkanes, salinity, and temperature on the interfacial composition and the structural parameters of the IL/O and W/O microemulsions were also investigated and discussed.