Changes in the chemical and chemical-structural composition of the organic matter of two different sewage sludges (aerobic and anaerobic) mixed with sawdust (1:1 and 1:3, v/v) during composting were determined by monitoring chemical and microbiological parameters as well as by pyrolysis-gas chromatography. Composting was carried out in periodically turned outdoor piles, which were sampled for analysis 1, 30, 60 and 90 days after the beginning of the composting process. Both volatile organic matter and the water soluble C fraction decreased during composting, indicating that the more labile C fractions are mineralized during the process. Microbial activity as measured by microbial respiration (CO(2) evolved from compost samples during incubation) also decreased with composting, reflecting the more stable character of the resulting compost. No major differences were observed between the four composts studied as regards their chemical-structural characteristics. The acetonitrile, acetic acid and phenol pyrolytic fragment tended to increase with composting. Although the final composts were more aromatic in nature than the starting materials, a low degree of humification was observed in all four composts studied, as determined by their high proportion of polysaccharides and alkyl compounds. For this reason, the relationship between pyrolytic fragments, such as benzene/toluene or benzene+toluene/pyrrol+phenols, which are used as indices of humification for soil organic matter, are not of use for such poorly evolved sludge composts; instead, ratios that involve carbohydrate derivatives and aromatic compounds, such as furfural+acetic/benzene+toluene or acetic/toluene, are more sensitive indices for reflecting the transformations of these materials during composting. Both the chemical and microbiological parameters and pyrolytic analysis provided valuable information concerning the nature of the compost's organic matter and its changes during the composting process.