This work analyzes two methods for preparing supported ionic liquid membranes using pressure or vacuum-based techniques. For this purpose, SILMs based on [bmim(+)][Cl-], [bminn(+)][BF4-], [bmim(+)][PF6-] or [bmim(+)][NTf2-] were characterized both fresh and after operation in a diffusion cell by scanning electron microscopy combined with energy dispersive X-ray and mass balance. In the case of the fresh impregnated supported ionic liquid membranes, it was found, that the highest amounts of ionic liquid immobilized within the respective membranes were achieved using the pressure method. Furthermore, the highest stability of the resulting supported ionic liquid membranes was also obtained with this method. The different behaviours of the membranes prepared under pressure or vacuum were correlated with the viscosity of the respective embedded ionic liquids. (C) 2009 Elsevier B.V. All rights reserved.