We investigated the aggregation behavior of polyoxyethylene (POE)-type nonionic surfactants in ionic liquid mixtures composed of 1-ethyl- and 1-hexyl-3-methylimidazolium tetrafluoroborates (emimBF(4) and hmimBF(4), respectively) by means of (1)H NMR chemical shift analysis and dynamic light-scattering measurements. The surfactants do not aggregate in hmimBF(4), whereas they are essentially immiscible with emimBF(4). That is, the surfactants are highly solvophilic to hmimBF(4), while are highly solvophobic to emimBF(4). In mixtures of emimBF(4) and hmimBF(4) micellization was observed. The critical micelle concentration (cmc) decreased and the mean hydrodynamic diameter of micelles, and hence, the micellar aggregation number, increased with increase in mole fraction of emimBF(4) in the ionic liquid mixture. (1)H NMR chemical shift analysis revealed that hmimBF(4) interacts with surfactant molecules preferentially in the ionic liquid mixture through interaction of hexyl groups with the surfactant hydrocarbon chains. The present work demonstrates that solvent quality can be controlled by mixing two ionic liquids to induce self-aggregation of amphiphilic molecules. (C) 2011 Elsevier Inc. All rights reserved.