In most applications, ionic liquids are used in multicomponent mixtures including molecular or other ionic compounds, sometimes nanoparticles or surfaces. Understanding the key factors at the molecular level that determine the various properties of ionic-liquid systems, and being able to predict them, are important research goals. The sophisticated nature of the interactions present in ionic liquids (coulombic and van der Waals forces, polarization and charge delocalization, flexibility and conformational richness) means they are not easily described by simple models, and thus their study using atomistic models and molecular simulation is both pertinent and timely. The main features of an ionic liquid structure are defined by charge ordering, as in molten salts of small, rigid ions. But a detailed picture shows that ionic liquids are more complex than high-temperature molten salts. Simulation and experiment show that some ionic liquids form segregated domains at the nanoscale: the non-polar alkyl side chains of the ions are excluded from a cohesive ionic network established between the groups of atoms that hold the electrostatic charges. The notion of the spatial domains provides a new way to understand solvation. Different solutes, according to their polarity or tendency to establish associative interactions, will be solvated in distinct local environments. Nonpolar molecules will tend to reside in the nonpolar domains. In the same manner as the nonpolar side-chains of the ions, these solutes will be excluded from the ionic network due to the cohesive energy of the charged groups. Dipolar solutes, e.g. acetonitrile, will interact closely with the charged headgroups but also with the nonpolar domains. This balanced behaviour makes acetonitrile an excellent solvent for many ionic liquids. Associating solutes, such as alcohols or water, will form H-bonds with the charged groups and will be solvated in the ionic domain. By spanning the entire composition range of a mixture between an ionic and a molecular liquid, different regimes will be found. Initially the molecular compound will be diluted, interacting with the domains of the ionic liquid. As its concentration increases, it will tend to aggregate on the domain towards which it has a greater affinity. At a point the molecular compound will cause a disruption of the ionic network. From then on the ionic liquid becomes the minority component, and form different sorts of aggregate (micelles or ion clusters) in the molecular solvent. The structure and interactions in mixtures of toluene with several ionic liquids will be presented and discussed in the light of the domain aggregation in the ionic solvents.. Fig.1 Simulation snapshot of toluene in [C 8 C 1 C 1 im][tf 2 N]. Alkyl chains of cations are in green, charged groups of cations and anions in red.