Surface tension and proton chemical shift data are presented for aqueous solutions of 1,3-dimethylimidazolium methylsulfate [C1mim][MeSO(4)], 1-butyl-3-methylimidazolium methanesulfonate [C4mim][MeSO(3)], and 1-hexyl-3-methylimidazolium chloride [C6mim]Cl, as well as surface tension data for 1-ethyl-3-methylimidazolium methanesulfonate [C2mim][MeSO(3)]. Even though a wide range of concentrations were investigated, none of the studied ionic liquid solutions displayed a plateau value for the surface tension, and a critical aggregation concentration (CAC) could not be established from the obtained surface tension data. However, further analyses of the chemical shift and surface tension data with the inclusion of additional available data for other ionic liquids revealed why CAC values in the literature may vary widely for principle and experimental reasons that are discussed in detail.