The effect of γ-cyclodextrin (γ-CD) on the monomer-dimer equilibria of 1-methyl-1′-alkylviologen radical cations (ClCnV·+: n = 1-10) has been studied by electrochemical and spectroelectrochemical techniques. The apparent dimerization is enhanced upon addition of γ-CD for n = 1-8 and the effect of γ-CD is a maximum for n = 4. This is attributed to the stabilization of dimers by the inclusion of the dimer in the cavity of γ-CD. However, the dimerization of C1C9V·+ and C1C10V·+, which exhibits strong tendencies of dimerization in the absence of γ-CD due to electrodeposition of the radical cations, is suppressed upon addition of γ-CD. From the dependence of the apparent dimerization constants on the concentration of γ-CD, the association constants of the dimer (KC2) and monomer (KC1) with γ-CD were determined for n = 1-8. Comparing the present results with our previous report with dialkylviologen radical cations (CnCnV·+: n = 1-5), we show that the dimers with the longer alkyl substituents in the same and opposite sides have little difference in their stability in the absence of γ-CD, but the former type of dimer is included preferentially in the cavity of γ-CD.