The stability of supported liquid membranes (SLMs) was partially improved by using a room-temperature ionic liquid (RTIL) as the membrane liquid and by employing a vapor permeation process for the separation of toluene/cyclohexane systems and the dehydration of aqueous I-propanol and aqueous ethanol mixtures. Because the basic characteristics of RTILs are that they have high surface tension and no detectable vapor pressure, the loss of membrane liquid by evaporation was avoided completely. The separation of toluene/cyclohexane mixtures was conducted for as long as 550 h, and the separation factor was kept within the range of 15-25, with the membrane showing good durability in vapor permeation. The use of SLMs with ionic liquids was shown to be a promising method for separating organic/organic and organic/water mixtures.