The results of ultrasonic absorption and conductance studies of 1:1 complexes of 2,6-O-dimethyl-beta-cyclodextrin (beta-DMCD) and cetyltrimetylammonium bromide (CTAB) at 25 degrees C are reported, CTAB was chosen as a guest species to eliminate any contribution to the experimental sound absorption from the monomer surfactant-micelle exchange process. The resulting sound absorption is considered to be primarily due to the 1:1 complex of beta-DMCD:surfactant, and its amplitude is observed to be smaller than that of beta-DMCD in aqueous solution, This suggests that the intermolecular interaction between the host and the guest species makes the structure of the 1:1 complex more rigid than that of beta-DMCD and, consequently, the degrees of freedom contributing to sound absorption are reduced, This is in contrast to the results observed for beta-CD:surfactant 1:1 complexes, for which it appears that the intermolecular guest/host interactions in the complex loosen the more rigid structure of beta-CD, which exists because of the presence of more intramolecular hydrogen bonding sites in beta-CD than in modified beta-CD. This increases the sound absorption of the complex relative to beta-CD.