This paper deals with the nature of the synergistic effect in flames of methane and formaldehyde mixtures with air. Combustion of mixtures of different fuels is of great practical and fundamental interest. It has been found that the addition of formaldehyde to a rich methane/air flame at a constant concentration of methane first reduces the flame speed and then begins to increase it. The synergism mechanism in this case is due to the predominant and complete consumption of formaldehyde due to its higher reactivity and its negative impact on the rate of methane consumption. The predominant combustion of one of the fuels leads to the existence of two spatially separated heat release zones in the flame. In the first zone, heat release is mainly due to the oxidation of formaldehyde and formyl radical, and in the second zone, it is due to the recombination of methyl radicals. An analysis of the flame speed sensitivity has shown that the key reactions affecting the flame speed are the stages of formation of radicals (mainly hydroxyl) or products that lead to their formation. Reactions that make the main contribution to the heat release generally do not affect the flame speed. It has been found that the interaction of two fuels CH4 and CH2O in a mixture with air leads to a marked increase in the superadiabatic temperature effect.