Reductive dechlorination of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in fly ash from municipal solid waste incinerator (MSWI) by sodium hypophosphite was carried out in a lab scale experiment. The MSWl fly ash with high concentrations of CaO and Fe2O3 and also trace metals such as Cu, Ph and Zn has the catalytic potential in dechlorinating PCDD/Fs at about 250-450 degrees C under oxygen deficient conditions. Sodium hypophosphite generates reducing agents (PH3 and H-2) at about 300 degrees C, which can increase the detoxification efficiency and accelerate the dechlorination reaction. The reductive dechlorination with 10% (w/w) sodium hypophosphite has shown high detoxification efficiencies of 99.4% and 99.3% at 350 degrees C and 450 degrees C, respectively, and as well as the dechlorination efficiencies about 98.7%. The reductive dechlorination is about the same efficiency as the melting process. In this study it is also shown that the bioassay method combined with HRGC/MS method as a reference is a feasible and low-cost technology for determining PCDD/Fs in environmental matrices. (c) 2006 Elsevier B.V. All rights reserved.