The effects that the solvent exerts on the end-capping process of a pseudorotaxane formed by the [Ru(NH3)(5)(4,4'-bpy)](2+) complex and beta-cyclodextrin were studied. In this process the 4,4'-bpy ligand acts as rigid axle and the cyclodextrin as ring or macrocycle. The stopper used was the [Fe(CN)(5)H2O](3-) Complex. The solvents used were mixtures of ethyleneglycol-water and tert-butyl alcohol-water. Results showed similar, although strange, behavior in both media studied. Thus, a decrease of the observed rate constant was obtained when the concentration of cyclodextrin increases for all the media studied. However, at fixed cyclodextrin concentrations, an increase of k(obs) was obtained when small quantities of the cosolvent were added to the medium and, further, a decrease of k(obs) for the higher quantities of the organic solvents. This strange behavior could be explained by taking into account electrostatic and specific solvent (solvent-solvent and solvent-solute) effects.