In the IR spectrum of the diethyl ether cation, an extraordinarily intense band, with an extremely broad bandwidth, was observed at 2700 cm(-1), much lower frequency than normal CH stretch frequencies. This band is assigned to the stretch band of the CH bond, which is hyperconjugated with the singly occupied molecular orbital of the oxygen atom. The hyperconjugation causes the delocalization of the a electron of the CH bond so that it enhances the acidity of the CH bond as well as the CH stretch band intensity. Theoretical simulation shows that the strength of hyperconjugation varies greatly with internal rotation of the ethyl group, and this is reflected in the large width of the observed CH stretch band. These results indicate that the DEE cation drastically changes its property from aprotic to highly acidic by the rotational isomerization of the ethyl group.