Heating a solution of the circular bis-p-phenylene-34-crown-10 and a dumbbell-shaped bipyridinium component, terminated at both ends by 4-R-phenyl-bis(4-tert-butyl-phenyl)meth stoppers, affords the corresponding [2]rotaxane when R is equal to H, Me, and Et, following the slippage of the macrocycle over the stoppers of the dumbbell. By contrast, no [2]rotaxane is obtained when R is equal to i-Pr. Computational investigations with the AMBER* force field provide an explanation of this dramatic substitutent effect. The phenomenon was simulated by the passage of the bis-p-phenylene-34-crown-10 macrocycle over four 4-R-phenyl-bis(4-tert-butyl-phenyl)methane model stoppers. For R equal to H, Me, Et, and i-Pr, there are two main energy barriers which have to be surpassed in order to permit the passage of the macrocycle over the bulky stoppers. When R is equal to H or Me, the rate-determining step is the passage of the macrocycle over a t-Bu-C6H4- ring. By contrast, when R is equal to Et or i-Pr, the rate-determining step becomes the passage of the macrocycle over the R-C6H4- ring. However, when R is equal to i-Pr, the resulting energy barrier is more than 21 kcal mol(-1) higher than in the case of any of the other stoppers. These results are in good agreement with the experimental observations and provide a quantitative explanation for the rigorous size complementarity requirements between macrocycle and stopper which have been observed experimentally.