For both economic and ecological reasons better knowledge of effects of incinerating waste on its persistent organic pollutant (POP) contents is needed. Thus, ash from three municipal solid waste incineration (MSWI) plants was collected and analyzed for elemental composition, carbon speciation and POP (PCDF, PCDD, PCN and PCB) contents. The ash was then subjected to two thermal treatments: a small batch treatment (3 g) in sealed glass ampoules and a large batch treatment (0.7 kg, in a kiln) under oxygen-deficient conditions. The POP contents of the ash (and the gas phase generated by the large batch treatment) were subsequently re-analyzed. Finally, principal component analysis of congener profiles were used to clarify the POPs' behavior in the treated ash. The results indicate that the thermal treatments had similar effects on PCDDs and PCBs, which apparently degraded but did not reform. They also had similar effects on PCDFs and PCNs, which were degraded but selectively reformed (both during and after the treatment). Furthermore, the ash composition did not significantly influence the homologue-specific congener patterns of the formed PCDFs and PCNs, but they had markedly lower chlorination degrees than those in the untreated ash and their overall toxicity was reduced by the kiln treatment, regardless of post-kiln concentrations and ash composition. (C) 2015 Elsevier B.V. All rights reserved.