Measurements by atomic absorption spectroscopy have been made of the concentration of magnesium in isothermal sets of hydrogen + oxygen + nitrogen flames. These measurements have led to values for the equilibrium constant of the reaction, Mg + OH ⇌ MgO + H, at 1570, 1800, 2030 and 2370°K, and to a value of 96 ± 3 kcal mole–1 for the bond dissociation energy of MgO. The method used to analyze the measurements has the advantage over previous flame photometric methods of not requiring a calibration of the atomizer or any values of spectroscopic parameters relating fractional absorption to metal concentrations. The amounts of hydroxides formed in these flames are negligible.