Between 0.4 and 2000 MHz the ultrasonic attenuation spectra and sound velocities of alpha-cyclodextrin solutions in potassium iodide/ water mixtures have been measured at different salt concentrations. The spectra reveal two relaxation terms, with relaxation times between 6 and 21 ns as well as 1.4 and 3 ns, respectively. The parameters of these terms are compared to those for aqueous solutions of mono- and disaccharides. The low frequency term is assigned to a conformational isomerization of the cyclodextrin truncated cone and to the formation of cyclodextrin-iodide inclusion complexes. The high frequency relaxation term is due to the exocyclic hydroxymethyl group isomerization. (c) 2006 Elsevier B.V. All rights reserved.