We studied the capillary wave spectra on the surface of an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([bmim][TFSI]), by using dynamic light scattering techniques. A transition from an over-damped to a damped oscillator behavior is observed with increasing temperature. We introduced an improved analysis method, which enables us to obtain the quantities, such as surface tension, surface excess entropy, and the viscosity, accurately and systematically as functions of temperature. We found that the surface dipole moment density for [bmim][ TFSI] is much smaller than that reported for [bmim][PF6]. (C) 2008 Elsevier B.V. All rights reserved.