Co-pyrolysis of biomass with polyethylene terephthalate (PET) was studied as a function of blend ratio and co-pyrolysis temperature by C-13 and H-1 solid-state nuclear magnetic resonance (NMR). The C-13 NMR spectra showed that upon heating to 400 degrees C in presence of the biomass, the formation of crystallites in PET was completely suppressed and that at higher temperatures there was increased formation and growth of the polycyclic aromatic hydrocarbons ( PAHs). This change in the PET degradation behaviour was attributed to the presence of radicals formed in char from biomass. The measurement of the H-1-T-1 relaxation enabled monitoring the changes in the concentrations of radicals formed, as a function of the blend ratios and the co-pyrolysis temperatures. It indicated that the increase in the radical concentrations correlated well with the increased degradation of the PET and growth of PAHs. (C) 2014 Published by Elsevier Ltd.