Cucurbiturils (CB) with seven and eight glycoluril units form inclusion complexes with 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4). These complexes have been characterized by the chemical shift changes in the 1H NMR and by the ESI-MS peaks. Due to complex formation, solubility of CB[7] and CB[8] in bmimBF4 is remarkably high (up to one order of magnitude higher than in water). In contrast, no evidence for complex formation was obtained in the case of CB[5] and CB[6]. Molecular modelling has provided a justification of these facts based on the relative molecular size of bmim+ and the corresponding dimensions of the CB capsule. Addition of minor quantities of CB[7] and CB[8] to bmimBF4 (in the order of 1-1000 molar ratio) is able to produce a remarkable influence on the viscosity of the ionic liquid. Also, the presence of low amounts of CB plays a strong influence on the performance of bmimBF4 as reaction medium for the Knoevenagel condensation of benzaldehyde and diethyl malonate catalyzed by NaOH. The results exemplify the potential of applying supramolecular chemistry to derive a generation of advanced ionic liquids specially tuned for a catalytic reaction. \copyright 2007 Elsevier B.V. All rights reserved.