Humic acids isolated from soils contaminated by polycyclic aromatic hydrocarbons (PAH) and from non-contaminated soils were investigated by analytical pyrolysis to examine the abundance and structure of soil bound PAH-residues. PAHs which are not extractable by organic solvents could be liberated from humic substances by analytical pyrolysis. Pyrolysates of humic acids derived from biodegradation experiments with PAH contaminated soils revealed a 2 to 8 fold enrichment of PAHs compared to those from non-polluted soils. This enrichment was also observed in humic substances isolated from contaminated field samples during a remediation experiment. Environmental implications for the formation of soil bound residues during bioremediation are discussed.