Hypothesis: Mixtures of the cationic surfactant hexadecyltrimethylammonium bromide (CTA-Br) and the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (bmim-BF4) in aqueous solutions are expected to behave as typical binary cationic surfactant system taking into account the surface activity displayed by the ionic liquid, instead of considering the IL as a water cosolvent. Experiments: Surface tension and conductivity measurements have been conducted as a function of the total concentration of the mixtures at different surfactant mole fraction (alpha(CTA-Br)) to investigate the surface active properties. Findings Turbidity immediately appearing when the compounds are mixed in water suggests the spontaneous formation of the low soluble compound hexadecyltrimethylammonium tetrafluoroborate (CTA-BF4), together with the salt formed by the respective counterions bmim(+) and Br- in solution. For alpha(CTA-Br) not equal 0.5, furthermore of the mentioned compounds, the spare bmim-BF4 (for alpha(CTA-Br) < 0.5) or CTA-Br (for alpha(CTA-Br) > 0.5), are also present in the aqueous solution. Systems containing excess of bmim-BF4 show a low critical aggregate concentration (cac), but an unexpected high surface tension at cac (gamma(cac) approximate to 53-56 mN/m), as pure CTA-BF4. For systems containing excess of CTA-Br, cac increases but gamma(cac) decreases up to 36 mN/m. Mixtures of pure CTA-BF4 and bmim-BF4 or CTA-Br behave as typical binary surfactant systems. (C) 2016 Elsevier Inc. All rights reserved.