An investigation on the behavior of SnO2 in the tin-bearing iron concentrates during the reduction sintering process (RSP) has been conducted by the authors' group. It was found that only about 30 wt.% of tin was removed under the optimal conditions. However, the underlying reason why it was so difficult to remove tin by the RSP was not understood. In this study, the phase evolution of tin oxide, iron oxide and calcium oxide under simulative sintering conditions was investigated by varying the gas compositions, sintering temperature and basicity (CaO/SiO2). The results indicated that the Ca-Sn oxides were prior formed at 900 degrees C-1000 degrees C in the reduction atmosphere with CO / (CO + CO2) = 10-40 vol.%, while the phases of Fe-Sn spinet (Fe3-xSnxO4) and Ca-Fe-Sn oxides were easily generated as the temperature increased to 1100 degrees C-1200 degrees C. The phases of Fe2-4x/2SnxO3 and Ca-Fe-Sn-Si oxides began to generate at 1100 degrees C in an oxidative atmosphere with O-2/(O-2 + CO2) = 25-100 vol.%. In addition, increasing alkalinity accelerated the formation of Ca-Sn oxides under reduction atmosphere, which brought about an adverse effect on the tin removal in the sintering process. (C) 2017 Elsevier B.V. All rights reserved.