The dilution method was used to research the interfacial composition, thermodynamic parameters, and structural parameters of [bmim][BF4] + Brij-35 + 1-butanol + toluene ionic liquid/oil (IL/O) microemulsion. The effects of the mole ratio of [bmim][BF4] to Brij-35 (R) and the addition of water on the above parameters were investigated. Studies of the interfacial composition revealed that cosurfactants at the interface (n(a)(i)) increased with the increase of R and decreased with the increase of additional water. This indicated that more cosurfactants were required to form a IL/O microemulsion than that of the traditional water/oil (W/O) microemulsion. The Delta G(t)degrees values of transfer process of cosurfactants from the continuous oil phase to the interfacial region were all negative, which suggested that the IL/O microemulsion formed spontaneously. The absolute values of Delta G(t)degrees increased with the increase of R and additional water. The results showed that the IL/O microemulsion with additional water formed more spontaneously and more stably. The Delta S-t degrees values were mostly positive, which implied that the formation processes of the IL/O microemulsions were entropy-driven. The greater number of cosurfactants at the interfacial layer contributed to form smaller ionic liquid droplets. Thus, smaller ionic liquid droplets were obtained with the increase in R, while larger droplets were obtained with the addition of water.