The density and sound speed measurements have been carried out for the binary systems consisting of tetra-n-butylammonium bromide (Bu4NBr) as a solute in benzene and CCl4 at T= 298.15 K (concentration range 0.01 mol . kg(-1) to similar to 0.8 mol . kg(-1)). The apparent molar volumes (phi(v)) and apparent molar isentropic compressibility (phi(Ks)) have been calculated and the values at infinite dilution are estimated. These data for Bu4NBr in benzene and CCl4 solutions are combined with the literature data of Bu4NBr in water to obtain transfer compressibility of Bu4NBr molecules from non-aqueous to aqueous solutions. The phi(v) behaviour as a function of concentration in benzene solutions indicates formation of reverse micelles and having critical micelle concentration (cmc) around 0.01x(2) of Bu4NBr (similar to 0.13 mol . kg(-1)). The compressibility data are further used to estimate hydration number of Bu4NBr in aqueous solutions by applying Shiio's method. The hydration number calculated for compressible organic electrolyte shows good agreement with the number obtained for clathrate hydrate of Bu4NBr using the chemical analysis and X-ray diffraction. (C) 2005 Elsevier Ltd. All rights reserved.