The effect of typical chemical impurities on the surface tension of two binary water ionic liquid (IL) systems was carefully evaluated for 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim] [BF(4)], and 1-butyl-3-methylimidazolium chloride, [bmim]Cl. The impurities considered were mineral acids and bases, potassium chloride, 1-methylimidazole, acetone, ethanol, vacuum grease, and a nonionic detergent. It is very likely that in particular detergents and/or vacuum grease may have been present in prior reported studies explaining the discrepancies present in the reported values for the particular cases of [bmim][BF(4)] and [bmim]Cl in water. As part of the study, the composition-dependent surface tension was remeasured three times for each for these two systems. For both binary water + IL systems, the results could be fitted to an exponential decay function, and the critical aggregation concentrations (CACs) were found to be in mole fraction of IL at 0.026 for [bmim][BF(4)] and 0.234 for [bmim]Cl.