Irradiation of sewage sludge reduces pathogens and can hydrolyze or destroy organic molecules. The effect of irradiation of sewage sludge on C and N dynamics in arable soil and possible interference with toxic organic compounds was investigated in soil microcosms using a clay soil. The soil was treated with phenanthrene and anthracene, with and without irradiated and non-irradiated sewage sludge amendment. All the treated soils were incubated for 182 days at 25 degrees C. The CO2 production and dynamics of inorganic N (NH4+, NO2- and NO3-) were monitored. Addition of sewage sludge (0.023 g g(-1) soil), anthracene or phenanthrene (10.0 microg g(-1) soil dissolved in methanol), and methanol (10 mg g(-1) soil) to soil had a significant effect on CO2 production compared to the control. However, there were no significant differences between soil treated with irradiated and non-irradiated sewage sludge. Irradiated sewage sludge increased the C and N mineralization of anthracene amended soils to a greater extend than in phenanthrene amended soils. Nitrification was inhibited for 28 days in soil treated with either methanol, anthracene and phenanthrene. Application of sewage sludge reduced such toxicity effects after 28 days incubation.