The chemical composition of organic matter (OM) in various fresh and composted wastes was characterized using pyrolysis-GC/MS. Composts made from source-separated biowaste, green waste, sewage sludge co-composted with green waste and municipal solid waste were collected from several French plants after four months of composting. Fresh non-composted wastes were analyzed in order to specify the origin of the pyrolysis products. The composition of the pyrolysates varied with the composted waste. The main pyrolysis products of bio- and green waste composts originated from lignin and polysaccharides. In biowaste and municipal solid waste compost, most cellulose-related structures originating from paper and cardboard were degraded after four months of composting. The presence of styrene di- and trimers in the pyrolysate of municipal solid waste compost was in agreement with the occurrence of plastics in this compost. Fatty acids were abundant in the pyrolysis products of sewage sludge and municipal solid waste compost, where they probably originated from greasy waste. Specific ratios of peak areas from pyrolysis products were calculated in order to describe the humification of OM, to confirm the presence of synthetic polymers, to evaluate the contribution of cellulose amongst polysaccharides and to assess the origin and degradation degree of lignin.