Conventional infrared absorption and Raman spectroscopy have been used to record the vapor phase spectra of methylpyridine-2-alphad(2),6-d(1), methylpyridine-2-alphad(2), and methylpyridine-3-alphad(2),-d(4) in the Delta upsilon (CH) = 1-4 regions. The spectra are analyzed with a theoretical model that takes into account, in the adiabatic approximation, the coupling between the internal rotation of the methyl group and the methyl CH stretching vibration. The principal parameters used in this model have been determined by ab initio calculations at the HF/6-31G** level of theory. A good agreement between experimental and calculated spectra is found. This indicates that this coupling is at the origin of the majority of the observed spectral profiles. A comparison of these results with those previously obtained for similar methylated molecules reveals that the change in type and size of the barrier to internal methyl rotation is at the origin of significant spectral differences. These changes are particularly important for methylpyridine-2-alphad(2), revealing that the methyl group experiences increasingly different internal dynamics with increasing energy. These spectral changes can be well explained by the deformation of the effective internal rotation potential in the vibrational excited states. The overtone spectra of the aryl CD stretching of methylpyridine-3-alphad(2),-d(4) have also been studied.