Abstract The influence of alkali and alkaline earth metals on the formation of hexavalent chromium (Cr(VI)) during municipal solid waste (MSW) incineration has drawn much attention due to the high toxicity of Cr(VI). In the present work, the distribution of Cr(VI) in MSW incineration ash residues sampled from various plants was measured. The results show that the oxidation of chromium in combustion process is determined by the content of free CaO (f-CaO) in the MSW and is facilitated by alkali metal compounds at lower temperatures in the flue gas cooling process. In some cases, the content of f-CaO is low during MSW incineration, and the oxidation of Cr2O3 in the case of low mole ratio of Ca and Cr was investigated. The mixture of CaO and Cr2O3 was heated at temperatures ranging from 973 to 1373K and the oxidation of Cr2O3 was detected by determining the consumption of oxygen in the carrier gas using an online mass spectrometer. According to the results, more chromium is oxidized in the thermally treated mixture at 1173K than at higher temperatures. At 1173K, part of chromium is oxidized to form CaCrO4 and the unoxidized chromium remains in the form of Cr2O3. In contrast, Cr2O3 is prior to form Ca3(CrO4)2 at higher temperatures and the unoxidized chromium is mostly transformed into CaCr2O4. Compared with Cr2O3, CaCr2O4 is more easily oxidized by CaO or K2CO3, indicating that more Cr(VI) would be formed during MSW incineration when trivalent chromium (Cr(III)) is transformed into CaCr2O4. Measures should be taken to suppress the formation of CaCr2O4 such as the control of the operating temperature and CaO addition.