Hydrothermal technology was investigated as a degradation method for polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in municipal solid waste incineration (MSWI) fly ash at 150, 200, and 250 degrees C under both non-oxidative and oxidative conditions. The results show that the removal efficiency of PCDDs/PCDFs in fly ash was accelerated by oxidative degradation and increased with an increasing temperature. The distributions of PCDD/PCDF homologues and isomers in fly ash were also analyzed after this hydrothermal process. There were no new isomers generated, and PCDD/PCDF degradation was not significantly dependent upon the position of the chlorine atoms. To further identify the PCDD/PCDF degradation pathways and avoid organic interfering substances in fly ash, octa-chlorinated dibenzo-p-dioxin (OCDD)-added aqueous solution degradation trials were conducted. All tetra- to octa-chlorinated PCDD and PCDF congeners were detected under both non-oxidative and oxidative atmospheres after OCDD-added decomposition. The formation of lower chlorinated PCDDs was attributed to a hydrodechlorination reaction, and those PCDF isomers arose as intermediates of OCDD degradation.