The transformation of organic matter during anaerobic digestion of mixtures of energetic crops, cow slurry, agro-industrial waste and organic fraction of municipal solid waste (OFMSW) was studied by analysing different samples at diverse points during the anaerobic digestion process in a full-scale plant. Both chemical (fiber analysis) and spectroscopic approaches ((13)C CPMAS NMR) indicated the anaerobic digestion process proceeded by degradation of more labile fraction (e.g. carbohydrate-like molecules) and concentration of more recalcitrant molecules (lignin and non-hydrolysable lipids). These modifications determined a higher degree of biological stability of digestate with respect to the starting mixture, as suggested, also, by the good correlations found between the cumulative oxygen uptake (OD(20)), and the sum of (cellulose+hemicellulose+cell soluble) contents of biomasses detected by fiber analysis (r=0.99; P<0.05), and both O-alkyl-C (r=0.98; P<0.05) and alkyl-C (r=-0.99; P<0.05) measured by (13)C CPMAS NMR.