Densities, conductivities, and polarity indexes of pyrene for aqueous solutions of a series of ionic liquids [C(n)mim] Br (n = 4, 6, 8, 10, 12) and [C(4)mim][BF4] have been determined at 298.15 K as a function of ionic liquid concentrations. It was shown that possible aggregation appeared for the ionic liquids in aqueous solutions except for [C(4)mim] Br. The critical aggregation concentration (CAC) of the ionic liquids, the ionization degree of aggregates (beta), the standard Gibbs energy of aggregation (Delta G(m)degrees), the limiting molar conductivity (Lambda(m)degrees), and the standard partial molar volume (V-m degrees) for the ionic liquids were derived from the experimental data. The dependence of the CAC, Delta G(m)degrees, Lambda(m)degrees, and V-m degrees on the length of the alkyl chain of the cations was examined. It was further suggested from volumetric data that a micelle was formed for [C(8)mim] Br, [C(10)mim] Br, and [C(12)mim] Br in aqueous solutions. Their apparent molar volumes at the critical micelle concentration (V-Phi,V-CMC), apparent molar volumes in the micelle phase (V-Phi(mic)), and the change of their apparent molar volume upon micellization (Delta V-Phi,V-m) were calculated by application of the pseudophase model of micellization. In addition, the average aggregation number of [C(n)mim] Br (n = 8, 10, 12) has been determined by the steady-state fluorescence quenching technique, and predicted from a simple geometrical mode. It is found that the experimental values are in good agreement with the predicted ones.