The interaction of 10-methylphenothiazine (MPT) and its electrochemically generated cation radical (MPT+.) with beta-cyclodextrin (beta-CD) in aqueous solution was studied by cyclic voltammetry, UV-vis absorption spectroscopy and circular dichroism. In aqueous solution, the increasing amount of beta-CD caused a negative shift in the anodic peak potential and an increase in the current of the MPT/MPT+. couple owing to the fact that more MPT was included in the beta-CD cavity. A series of solutions in a methanol and water mixture were investigated, and fast scan cyclic voltammetry, which showed that the irreversible wave became a quasi-reversible wave in the presence of beta-CD, confirmed the stabilization of the MPT+. intermediate by beta-CD. UV-vis absorption spectroscopy and circular dichroism, were also performed to study the inclusion phenomena. The electrochemical and absorption spectral data indicated a 1:1 inclusion complex formation of beta-CD with the cation radical as well as the parent MPT, resulting in enhanced stabilization of the cation radical in the microscopic environment of the cyclodextrin cavity with respect to the consumptive homogeneous chemical reaction. The estimated formation constants of MPT + beta-CD and MPT+. + beta-CD systems were 90.3 mol(-1) 1 and 3205 mol(-1) l respectively. Moreover, the reason for the unusually high stability of the MPT+. beta-CD complex was discussed in the light of the dipole-induced dipole interaction and its conformation change.