Wood biodegradation is carried out mainly by white-rot and brown-rot fungi. These microorganisms secrete extracellular enzymes that are able to degrade the wood cell walls to carbon dioxide and water. Both scientific and commercial interests have generated a need to develop an in situ analytical test able to assess the level of cell wall biodegradation. In this paper wide-line solid state nuclear magnetic resonance (NMR) spectroscopy has been applied to this problem. An NMR method has been developed based on proton spin-lattice (T-1) and spin-spin (T-2) relaxation time measurements on wood samples subjected to fungal attack. Beech (Fagus sylvatica) and pine (Pinus sylvestris) wood degraded for 6 weeks by the white-rot fungus Coriolus versicolor and by the brown-rot fungus Coniophora puteana showed a significant increase in the NMR relaxation times as the biodegradation took place. T-1 showed increase up to 8.8-fold for white-rotted samples and 1.5-fold for brown-rotted samples. T-2 showed an increase of up to 2.1-fold for white-rotted samples and 1.7-fold for brown-rotted samples. Statistical analysis of the data (t-test) confirmed the significance of these variations. Such variations in relaxation times as a result of biodegradation are interpreted as being due to an increase in mobility of the cell wall’s molecular components and of the water present in the cell walls, as a consequence of the oxidative breakdown by the action of fungal enzymes. In all samples white-rot fungi showed a more marked degradative power than brown-rot on both beech and pine wood. This finding offers the opportunity to develop a nondestructive test able to assess the level of degradation in wood samples.