Ultrasonic attenuation spectra as well as dependencies upon surfactant concentration of the specific electric conductivity, the sound velocity, and the density have been measured for aqueous solutions of alpha-cyclodextrin with different amounts of n-decyltrimethylammonium bromide added. In addition to cyclodextrin isomerization the spectra reveal cyclodextrin-surfactant complexation and, above a critical association concentration, the monomer exchange between surfactant micelles and the suspending liquid. The data are evaluated assuming most alpha-cyclodextrin to form complexes, predominantly with 1:1 but also with 2:1 cyclodextrin-surfactant stoichiometry. (c) 2008 Elsevier B.V. All rights reserved.