Polychlorinated biphenyls (PCBs) were widely used in the past and now contaminate many industrial and natural areas. In this study, a PCB-contaminated soil from a former paper mill was mixed with a yard trimmings amendment and composted in field scale piles to determine the effect of soil to amendment ratio on PCB degradation. Temperature, oxygen concentrations, and a number of other environmental parameters that influence microbial activity during composting were monitored. The PCBs in contaminated soil had a concentration of 16 ±1 mg/kg dw and an average of 4 chlorines per biphenyl. The soil was composted with five levels of yard trimmings amendment (14% to 82% by weight) in pilot scale compost piles (25 m3) turned once per month. Results showed up to a 40% loss of PCBs with amendment levels of 60% and 82%. Congener specific PCB analysis indicated that less chlorinated PCB congeners (1-3 chlorines per biphenyl) were preferentially degraded. Bench-scale studies indicated that less than 1% of the PCBs in the contaminated soil were volatilized from composts during incubation with forced aeration at 55°C. In conclusion, PCB loss observed during the composting of the PCB-contaminated soil appeared to be largely due to biodegradation and not volatilization. Effective bioremediation of aged PCB-contaminated soils may require coupling of composting with additional remediation technologies to reduce levels of PCB congeners with greater than 4 chlorines.