The impact of a constant uniform magnetic field on equilibrium flame temperature under constant volume conditions was examined. The equilibrium combustion compositions were found using an expression for Helmholtz free energy that included a magnetic-field contribution. Changes in the Helmholtz free energy for a mixture of paramagnetic and diamagnetic ideal gases were minimized using the method of Lagrange multipliers. The magnetic susceptibility for the paramagnetic gases was calculated using the Curie law. Equilibrium flame temperatures were obtained from the first law of thermodynamics for a closed adiabatic system with no work interactions. A model reaction of methane in air was used to quantitatively examine changes in the equilibrium flame temperature and product mole fractions as a function of volume and magnetic induction. Equilibrium flame temperature and product mole fractions were plotted as a function of magnetic-field strength and volume for all product species. The results indicate that with the increase in magnetic-field strength, the equilibrium flame temperature increases for all volumes considered.