[en] Dielectric spectroscopy as well as ²H and ³¹P nuclear magnetic resonance spectroscopy (NMR) are applied to probe the component dynamics of the binary glass former tripropyl phosphate (TPP)/polystyrene (PS/PS-d₃) in the full concentration (c_TPP) range. In addition, depolarized light scattering and differential scanning calorimetry experiments are performed. Two glass transition temperatures are found: T_g1(c_TPP) reflects PS dynamics and shows a monotonic plasticizer effect, while the lower T_g2(c_TPP) exhibits a maximum and is attributed to (faster) TPP dynamics, occurring in a slowly moving or immobilized PS matrix. Dielectric spectroscopy probing solely TPP identifies two different time scales, which are attributed to two sub-ensembles. One of them, again, shows fast TPP dynamics (α₂-process), the other (α₁-process) displays time constants identical with those of the slow PS matrix. Upon heating the α₁-fraction of TPP decreases until above some temperature T_c only a single α₂-population exists. Inversely, below T_c a fraction of the TPP molecules is trapped by the PS matrix. At low c_TPP the α₂-relaxation does not follow frequency-temperature superposition (FTS), instead it is governed by a temperature independent distribution of activation energies leading to correlation times which follow Arrhenius laws, i.e., the α₂-relaxation resembles a secondary process. Yet, ³¹P NMR demonstrates that it involves isotropic reorientations of TPP molecules within a slowly moving or rigid matrix of PS. At high c_TPP the super-Arrhenius temperature dependence of τ₂(T), as well as FTS are recovered, known as typical of the glass transition in neat systems