In this study, we explore the effect of magnesium content on the ability of reactive nanocomposite foils to generate heat, by comparing three chemistries: Al:Zr, Al-8Mg:Zr, and Al-38Mg:Zr. These correspond to foils with alternating aluminum and zirconium layers where the Al is either pure, an 8 at%Mg alloy, or a 38 at.%Mg alloy, respectively. Measurements performed in a specially designed bomb calorimeter show that Al-8Mg:Zr foils perform the best, generating the greatest gravimetric heat in air, oxygen, and nitrogen environments. Both Mg-containing foils release a visible plume of particles and vapor upon reacting, which was recorded with a high speed camera. This ejected mass includes Mg vapor and particles of all three metals. Both the vapor and particles oxidize rapidly in air, resulting in single metal-oxide particles. The reacted foils, particularly the Al-8Mg:Zr samples, contain voids and higher levels of oxygen and nitrogen throughout their thicknesses than reacted Al:Zr foils. To explain the higher heats of reaction for the Al-8Mg:Zr foils, we suggest that the out-diffusion and evaporation of Mg generates a high concentration of vacancies that enhance oxygen and nitrogen diffusion throughout the foil, thereby increasing the degree of oxidation and nitridation. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.