The phenomenon of superadiabatie flame temperature that was numerically predicted and experimentally observed in rich acetylene/oxygen flames used in chemical vapor deposition of diamond layers is of fundamental interest to combustion science. The chemical mechanism of this phenomenon has not been systematically studied. In this paper, the structure of planar freely propagating premised flames of mixtures of CH4/air, CH4/O-2, C2H2/H-2/O-2, C2H4/O-2, C3H8/O-2, and H-2/O-2 was computed using detailed chemistry and complex thermal and transport properties to gain an insight into the common chemical mechanism responsible for this phenomenon in these flames. It was found that superadiabatic temperatures occur only in hydrocarbon flames when the equivalence ratio of the mixture is greater than a critical value, but not in hydrogen flames. The superadiabatic temperature in these hydrocarbon flames is associated with superequilibrium concentrations of some hydrocarbon species and H2O, and with negative heat production rates at the end of main heat release reactions. The net negative heat production is caused by the endothermic dissociation reactions of these superequilibrium larger hydrocarbon molecules into smaller ones and of H2O into H-2 and OH. In hydrogen flames, neither the concentration of H2O nor the flame temperature exceeds their superequilibrium level. Radiation heat loss has negligible effect on the peak flame temperature and therefore does not affect the occurrence or the degree of superadiabatic temperatures.