The fibre present in mushroom compost samples was separated by a standard method of selective hydrolysis into three components : acid detergent fibre (ADF), neutral detergent fibre (NDF) and lignin. Results of differential thermogravimetric (DTG) analysis of these fractions suggest that with NDF the matrix polysaccharides were detected as a shoulder to the cellulose peak and that structural hemicelluloses pyrolysed before the lignin decomposition peak. However, in ADF, only cellulose and lignin pyrolysis peaks were detected and a hemicellulose decomposition peak or shoulder was not present. In the lignin there were two pyrolysis peaks which represented residual polysaccharides and lignin. Thermograms of humus which had been separated from the compost by ultrasonic treatment showed that microbial polysaccharides and phenolic compounds were the main components. Lignin and humus contained high concentrations of nitrogen compared with the other fractions. Comparison of thermograms of uncomposted wheat straw and straw from compost showed that straw cell-wall components were altered by fermentation.