One strategy to suppress metal vapor production in fully dense thermite mixtures is to dilute with excess inert metal to lower the reaction temperature below the boiling point of the metal. However, if the diluent is inhomogenously distributed throughout the microstructure, localized hot regions can be present, resulting in local temperatures exceeding the boiling point of the metal. Fully dense Al:Cu2O:Cu foils are analyzed to determine the critical length scales necessary to suppress Cu vapor during reaction. The microstructures of these foils, more specifically length scales of reactive and inert material, are critically analyzed using image analysis. The results from image analysis are then compared with finite element heat transfer simulations used to determine critical length scales of thermite and diluent required to suppress metal vapor production during reaction of fully dense Al:Cu2O:Cu foils. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.