The reaction Mg + O₃→ MgO + O₂ has been studied by the pulsed photodissociation at 193.3 nm of magnesium acetyl acetonate [Mg(C₅H₇O₂)₂] vapour to produce Mg atoms in an excess of O₃ and N₂ bath gas, followed by time-resolved laser-induced fluorescence (LIF) spectroscopy of atomic Mg at 285.2 nm [Mg(3¹ P₁–3¹S₀)]. The resulting rate coefficient is given in the Arrhenius form by k(196 < T/K < 368)=(2.28 ± 0.74)× 10^–10 exp[–(139 ± 84 K)/T] cm³ molecule^–1 s^–1. This reaction is therefore the most rapid oxidation process of atomic Mg in the atmosphere between 65 and 95 km. The reaction MgO + O₃→ MgO₂+ O₂ was investigated by the pulsed photodissociation of MgO₃ coupled with time-resolved LIF by pumping the MgO(B ¹Σ⁺–X ¹Σ⁺, Δv= 0) transition at 499.4 nm and monitoring emission from the MgO(B ¹Σ⁺–A ¹Σ⁺, Δv⩽ 0) transition at wavelengths greater than 600 nm. This yields k(217 < T < 366 K)=(2.19 ± 1.8)× 10^–10 exp[–(548 ± 271 K)/T] cm³ molecule^–1 s^–1. Ab initio quantum calculations were used to show that the most stable form of magnesium in the upper atmosphere is Mg(OH)₂, formed from a rapid recombination reaction between MgO and H₂O. A one-dimensional (1D) model of magnesium was then constructed by using observed Mg⁺ profiles above 85 km to constrain both the chemical reaction scheme and the meteoric input flux of the metal. The atomic Mg layer is predicted to occur at the same height as the Na layer (ca. 90 km), but with a peak concentration that is smaller by a factor of ca. 5. The model indicates that the ratio of the Mg/Na flux from meteoric ablation is about 0.5. This is strong evidence that the available mass of meteoroids does not ablate completely and that the residual fraction is enriched in magnesium.