The fate of high molecular weight and medium molecular weight chlorolignins (MW > 10K and MW > IK but < 10K, respectively) in aerobic soil under in vitro conditions was studied. After several days of incubation, less than 5% of the chlorolignin absorbance could be recovered by water extraction. This indicates a rapid immobilization into the soil. About 11% of the organic chlorine content of chlorolignins was mineralized to chlorides at both 1000 and 2000 ppm concentrations after 90 days of incubation in soil. GC-ECD and GC-MS analysis of organic solvent extracts of soil incubated with chlorolignins (1000 ppm) showed no detectable accumulation of low molecular weight chlorinated compounds. There was no significant difference in viable bacterial counts between control soils and those incubated with chlorolignins. Aqueous extracts of soils incubated with chlorolignins showed no toxicity above the detection limit of the Microtox system. Organic solvent extractions recovered toxicity more completely. Since all these toxicities are very low (EC50 > 100 g of soil/L), it was concluded that chlorolignins do not release significant toxicity during incubation in the soil.