A thermodynamic study of a gemini type surfactant, bis(dodecyl dimethylammonium)diethyl ether dibromide (12-EO1-12), has been carried out in water and in the presence of beta-cyclodextrin (beta-CD) at 298 K. Density and sound velocity data for the aqueous solution of the gemini in both systems were analyzed to calculate the molar apparent and partial volumes and adiabatic compressibilities. For the ternary system, the shift of the critical micelle concentration and the values of the transfer properties prove the formation of a molecular complex with a predominantly 1:2 stoichiometry (one surfactant molecule per two CDs). (C) 2007 Elsevier B.V. All rights reserved.