The organic compositions of five different sludges and their residues obtained by neutral detergent extraction (NDF) were characterized by pyrolysis-GC/MS, and the composition of their “soluble” fractions was deduced. These organic characteristics were used to explain the observed mineralization when the sludges were added to a soil under controlled conditions. Certain of the biochemical compounds revealed by pyrolysis-GC/MS, such as N-containing compounds, ketones and polysaccharides, were common to all five sludges, but in different proportions. Lignin-derived products were, in contrast, only present in two sludge samples, and were especially abundant in the sludge composted with green wastes and wood pieces. Aliphatic compounds and acids probably originating from lipids were only observed in some of the pyrograms. Although the compositions of the different NDF varied, some global trends were apparent. The sludge NDF pyrolysates contained compounds originating from lignins, cellulosic and non-cellulosic material. Some of the proteins and peptides evidenced by pyrolysis in the total sludge remained after extraction in some residues, although they were expected to be solubilized in neutral detergent. The soluble residue consisted mainly of simple sugars, proteins, peptides and lipids. However, the solubilization of some specific compounds varied with the global composition of the sludge. No specific recalcitrant compounds could be evidenced in the neutral detergent extract. Relationship were established between relative abundance of some compounds and C and N mineralization rates: C mineralization rate after 14 days of incubation was correlated with the cumulated relative areas of (ketones + aliphatics + acids), C mineralization rate at day 168 with those of (phenol + benzene) and N mineralization rate with those of ketones, and N-containing compounds.