Imidazolium ionic liquids are generally regarded as optically transparent materials. Herein, we report on the optical behaviour of a typical imidazolium ionic liquid that not only suggests that [bmim][PF6] does have significant absorption in the UV region, but it also brings into light a very interesting fluorescence behaviour. The fluorescence peak position of [bmim][PF6] is found to be very strongly dependent on the excitation wavelength. The unusual fluorescence response of [bmim][PF6] is attributed to the presence of several energetically different associated forms of the imidazolium ions in the ionic liquid. (C) 2004 Elsevier B.V. All rights reserved.