The photoexcited triplet state of a 4-4'-disubstituted dithiophene has been investigated by time-resolved electron paramagnetic resonance in two different solid matrixes-glassy toluene which acts as an amorphous isotropic medium and a spirocyclophosphazene inclusion compound-which provides an ordered confining structure to the guest molecules. Different spectral line shapes with different temperature dependence have been obtained using continuous-wave or spin-echo detection. This behavior was attributed to spin relaxation due to modulation of the zero-field splitting tensor induced by fast librational motion of the dithiophene triplet. Moreover, differences between line shapes in the two matrixes were reproduced considering a librational motion occurring preferentially around different molecular axes.