The photoexcited triplet state of a 4-4‘-disubstituted dithiophene has been investigated by time-resolved electronparamagnetic resonance in two different solid matrixesglassy toluene which acts as an amorphous isotropicmedium and a spirocyclophosphazene inclusion compoundwhich provides an ordered confining structureto the guest molecules. Different spectral line shapes with different temperature dependence have been obtainedusing continuous-wave or spin−echo detection. This behavior was attributed to spin relaxation due tomodulation 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 librationalmotion occurring preferentially around different molecular axes.