We investigate the molecular dynamics in the binary glass forming system methyltetrahydrofuran (M-THF) and tristyrene. Although the components are miscible in the full concentration and temperature range, two glass transitions can clearly be distinguished in differential scanning calorimetry. We selectively probe the reorientational dynamics of M-THF and tristyrene by means of dielectric spectroscopy and depolarized dynamic light scattering, respectively. While, apart from the observed plasticizer effect, the motion of the larger molecules remains almost unchanged, it is shown that the smaller M-THF molecules take part in both glass transitions. Moreover, below the upper T(g) of the mixture, the remaining mobile M-THF molecules clearly show confinement effects in their relaxation behavior. In order to elucidate the nature of the observed secondary relaxation processes, we first characterize the influence of the methyl group of M-THF on the dynamics in the mixtures by comparing the results obtained so far with the relaxation behavior observed in blends of THF and tristyrene. Finally, we employ (2)H NMR spectroscopy to clarify the nature of the secondary relaxations of THF-d(8) in the latter mixtures and conclude on the basis of the NMR and dielectric results that the high-frequency wing observed in neat M-THF appears as a genuine Johari-Goldstein β-relaxation in the mixtures, whereas the faster secondary process is due to internal degrees of freedom of the nonrigid THF ring.