Speed of sound (u) studies for the ternary systems formed by beta-cyclodextrin (beta-CD) or 2,6-di-o-methyl-beta-cyclodextrin (DI-MEB), dodecyltrimethylammonium bromide (D12TAB), and water are reported at 298.15 K. Measurements have been carried out (I) as a function of [D12TAB] in the presence of different initial concentrations of beta-CD or DIMEB, (II) as a function of [beta-CD] for various initial micellar solutions of [D12TAB], and (III) as a function of the concentration of the inclusion complexes beta-CD:D12TAB and DIMEB:D12TAB. From these data, the microencapsulation process of the surfactant monomers into the cyclodextrin cavities and its effect in the micellization process of the surfactant have been carefully analyzed. The apparent critical micellar concentration (cmc*) of D12TAB is found to increase linearly upon the addition of cyclodextrin, while the free surfactant concentration [D12TAB]f available for the micellization process in the micellar region slightly decreases in the presence of beta-CD or remains basically constant in the presence of DIMEB. The inclusion complexes formed (beta-CD: D12TAB and DIMEB:D12TAB) present stoichiometries of 1.1: 1 and 1:1, respectively, confirming the usual 1:1 scheme of this kind of compound. (C) 1994 Academic Press, Inc.