The coupling of two large amplitude motions, the internal rotation of the methyl group and the intramolecular proton transfer, has been investigated for jet-cooled 5-methyltropolone, 5-methyltropolone-OD, and the 5-methyltropolone-(H2O)(1) 1:1 hydrogen-bonded complex by measuring the fluorescence excitation, dispersed fluorescence, and hole-burning spectra in the S-1-S-0 region. The vibronic bands in the excitation spectrum of 5-methyltropolone consist of four components originating from the transitions between the sublevels in the S-1 and S-0 states. The intensity of the bands, the frequencies, and the change in the stable conformation of the methyl group upon photoexcitation have been analyzed for 5-methyltropolone-(H2O)(1) by calculating the one-dimensional periodic potential function, which provides the correlation between the internal rotational levels of 5-methyltropolone-(H2O)(1) and the sublevels of 5-methyltropolone. It has been shown that the electronic transitions between the sublevels within the same symmetry are allowed in 5-methyltropolone. The tunneling splitting of the zero-point level in the S-1 state is 2.2 cm(-1) for 5-methyltropolone. The corresponding splitting for 5-methyltropolone-OD is less than 0.5 cm(-1). A drastic decrease of the tunneling splitting for 5-methyltropolone as compared to that for tropolone (19.9 cm(-1)) is ascribed to a strong coupling between the two large amplitude motions in the S-1 state. The existence of a similar coupling has been suggested in the S-0 state of 5-methyltropolone. The excitation of the sublevel in the S-1 state considerably promotes proton tunneling. This effect has been explained by the delocalization of the wave function of the internal rotation of the methyl group. The two-dimensional potential energy surface along the proton transfer coordinate and the rotational angle of the methyl group has been calculated to explain the effects of the coupling on proton tunneling.