Surface of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) + water mixture is investigated using surface tension measurement and sum-frequency generation spectroscopy. The liquid surface is mostly covered by the [BMIM]+ cations at very low bulk concentration. An unusual increase in surface tension from mole fraction c approximate to 0.016 up to approximate to 0.05 suggested that the BF4- anions start to appear at the surface from c approximate to 0.016 until the anions and cations are equally populated at c approximate to 0.05 or higher. From the analysis of the spectra, the terminal methyl group of the butyl chain in the cation is polar-oriented with its symmetry axis aligning rather vertical to the surface for the whole range of concentration. (c) 2005 Elsevier B.V. All rights reserved.