Surface-active ionic liquids based on imidazolium cations are promising targets for micellar catalysis in aqueous solution, yielding enhanced rate constants compared to surfactants based on n-alkyltrimethylammonium cations and exhibiting a pronounced counterion dependence (Bica Chem. Commun. 2012, 48, 50135015; Cognigni Phys. Chem. Chem. Phys. 2016, 18, 1337513384). Probably most relevant to these effects is the interplay between headgroup hydration and counterion binding. To obtain more detailed information on these effects, aqueous solutions of 1-dodecyl-3-methylimidazolium ([C12MIM]) bromide, iodide, and triflate (TfO) were investigated at 45 C using broadband dielectric spectroscopy, viscosity measurements, and small-angle X-ray scattering experiments. Effective hydration numbers were determined, and information on the locations and mobilities of the condensed counterions, X, was derived. It was found that [C12MIM] halide micelles were less hydrated than the corresponding n-dodecyltrimethylammonium ([C12TA]X) aggregates. Together with their somewhat weaker counterion condensation, this difference probably explains their higher catalytic activity. Whereas [C12MIM]Br micelles remained roughly spherical in the studied concentration range, rodlike aggregates were formed at high concentrations of the iodide and, in particular, the triflate surfactants. It appears that the much lower mobility of condensed TfO counterions is the reason for the very low catalytic activity of [C12MIM]TfO micelles.