In this study, the dependence of the flammable concentration on particle size is investigated for Phase Change Material (PCM) and Encapsulated Phase Change Material (EPCM) particles using a novel continuous particle dispersion apparatus into which a propane flame is introduced creating a test akin to the flash-point test for liquids. The results show that the threshold concentration is a strong function of particle size. For tested particles with size ranging from 290 mu m to 750 mu m, the threshold concentration is above the predictions based on an instantaneous heat transfer limit, and is approximately linear with the particle size, following a heat transfer limited ignition model. For sizes above approximate to 1 mm, the particles behave like the bulk material, and ignition is not observed for the concentrations tested. The results obtained here are important for the safe construction, handling, and operation of systems using PCM and other particles. (C) 2016 Elsevier Ltd. All rights reserved.