The changes at molecular level induced by cold argon plasma treat-ments on fibers obtained from chemi-thermo-mechanical pulp (CTMP) fibers were investigated. The radicals formed on CTMP fibers after treatments were identified and quantified by Electron Paramagnetic Resonance (EPR) spectroscopy. The plasma conditions which maximize the formation of radicals on fibers were assessed: after treatment with 0.4 mbar Ar pressure and 75 W radiofrequency power, phenoxy radicals triple their concentration in only 60 s and reach a value 4 times higher than that reported for laccase-catalyzed lignin oxidation. It was found that in plasma-treated fibers, the formation of radicals competes with their coupling. This latter result leads to cross-linkages of the lignin mono-meric units and formation of new intermonomeric C-C and C-O bonds, for the first time assigned to specific molecular interactions through Heteronuclear Single Quantum Coherence (2D-HSQC) spectroscopy and Nuclear Magnetic Resonance spectroscopy of carbon (13C-NMR). These results were confirmed by Nuclear Magnetic Resonance spectros-copy of phosphorous (31P-NMR). The lack of evidences of inter-fiber bond interactions, deduced from Gel Permeation Chromatography (GPC) data, suggests the possible application of plasma treatments for the production of wood fiber-based composites
