The interfacial composition, thermodynamic properties and structure parameters of the W/O microemulsion systems containing alkyl-3-methyl imidazolium bromide (C n mimBr, n = 12, 14, 16)/1-pentanol/n-octane/aqueous NaCl (HCl or NaOH) have been studied by a dilution method. The molar solubility of 1-pentanol in oil phase (X o a ) and the molar fraction of 1-pentanol at the interfacial layer (X i a ) were obtained from the dilution equation of the W/O microemulsions. Furthermore, the distribution constant K of 1-pentanol between the interfacial layer and the bulk oil phase was determined, along with the standard Gibbs’ free energy change of transferring alcohol from the continuous oil phase to the interfacial layer, –ΔG o o→i . Some structure parameters related to the microemulsion droplets were also calculated. The effects of NaCl, HCl and NaOH on the properties and structures of the W/O microemulsions were also explored. It was found that as the content of the aqueous NaCl (HCl and NaOH) increases, X o a and X i a would decrease, the formation of the microemulsion would be more thermodynamically spontaneous, and the radius of the water pool significantly increases.