The origin of the variation in the regioselectivity of the nucleophilic ring opening of a series of bicyclic aziridinium ions derived from N-alkylprolinols was investigated by quantum-chemical computations (M06-2X/6-31+G(d,p)-SMD). These aziridiniums differ only in the degree and the configurations of F-substitution at C(4). With the azide ion as nucleophile, the ratio of the piperidine to the pyrrolidine product was computed. An electrostatic gauche effect influences the conformation of the adjoining five-membered ring in the fluorinated bicyclic aziridinium. This controls the regioselectivity of the aziridinium ring opening.