A detailed description of the thermal relaxation processes in MEH-PPV is reported. Bulk methods such as DMTA were employed in conjunction with other techniques that probe molecular motions. such as fluorescence spectroscopy, thermal stimulated current. and C-13 NMR. From the two main transitions observed (glass transition process at 340 K and beta-relaxation between 200 and 220 K). it was demonstrated that the first is strongly correlated with the dissociation of a fluorescent emissive interchain complex and that the second relaxation involves movements of the lateral substituents of the polymer backbone and, more specifically, their CH2 groups. NMR dipolar chemical shift correlation experiments pointed an increasing gain in mobility through the side chain. the lateral carbons close to the aromatic ring being more rigid than those located more distant from be main polymer chain. A kinetic model involving the dissociation of interchains to re-form intrachain excitons was proposed to explain the profiles of the photolumineseence spectra at higher temperatures.