Chemical changes of polysaccharides and lignin in Prunus armeniaca decayed by the ascomycete fungus Hypocrea sulphurea were investigated. Solid-state 13C NMR spectra showed that polysaccharides were the main components of fresh and decayed wood. Decomposition of cellulose and hemicellulose by the fungus was minimal although a slight preference for crystalline as compared with amorphous cellulose was recognised. Comparison of the signal intensity of the resonance at 145 ppm in fresh and decomposed wood suggested that the fungus had decomposed tannin constituents. Thermochemolysis with tetramethylammonium hydroxide (TMAH) and product identification by gas chromatography–mass spectrometry revealed that the ratio of syringyl-type (S) units to guaiacyl-type (G) units decreased from 1.8 to 1.1 following fungal attack. Increases in both guaiacyl and syringyl acid-aldehyde ratios (Ad/Al)G, (Ad/Al)S together with an increase from 0.82 to 3.54 in the ratio of methyl 3,4-dimethoxybenzoate to the sum of 1-(3,4-dimethoxyphenyl)-1,2,3-trimethoxypropane (threo and erythro-isomers) ΓG from the decayed wood confirmed oxidative Cα–Cβ cleavage for the mechanism of lignin decay by this ascomycete.