A numerical study with a detailed chemistry mechanism has been conducted to investigate the chemical effects of steam on the temperature in methane oxy-steam combustion; steam is used to moderate the high flame temperature produced by combustion of the fuel using oxygen. The temperature profiles of the oxy-steam combustion are consistent with those of traditional combustion when the mole fraction of steam in the oxidant is 72.5%. The comparisons between the maximum temperatures when steam is added and the corresponding artificial material X addition reveal that when the mole fraction of H2O (or X) is between 0.60 and 0.66, the overall chemical effects of the steam peak. High steam concentration in the atmosphere enhances the three-body reaction, raises the concentration of OH, and lowers the concentration of H during oxy-steam combustion, in contrast to traditional combustion. Consequently, when the steam's mole fraction is between 0.5 and 0.7, the three-body reaction (H + O-2 + H2O double left right arrow HO2 + H2O) (R35) is the key elementary reaction that determines the combustion temperature during oxy-steam combustion; this reaction also provides sufficient HO2 to R287, giving R287 the highest heat release rate. When the steam mole fraction exceeds 0.75, the reduced oxygen mole fraction enhances the rate of CO production. Concurrently, R38 is the key elementary reaction for determining the combustion temperature because it supplies a large amount of radical OH species. Therefore, R99 dominates R287 and has the highest heat release rate. When the steam mole fraction is 0.725, R35 and R38 play a very important role affecting the combustion temperature; R99 makes the largest contribution to temperature, while R287 and R43 also make significant contributions to the temperature. (C) 2014 Elsevier Ltd. All rights reserved.