Reliable thermal property data are necessary to improve the fidelity of chemical hydride thermal decomposition models. The thermal diffusivity and conductivity of ammonia borane (NH3BH3) and its partial thermolysis product (polyiminoborane) were measured at various packing densities using a transient plane source technique under ambient conditions. The particle size of the ammonia borane powder was between 200 and 600 mu m, while the particle size of the polyiminoborane powder was between 10 and 30 mu m. The thermal diffusivity and conductivity of the ammonia borane increased from 0.17 to 0.24 mm(2)/s and 0.19 to 0.44 W/m K (+/-10%), respectively, when its packing density was increased from 0.37 to 0.58 g/cm(3). The increase in thermal conductivity is due to the increase in contact area between particles and the increase in the thermal diffusivity is related to an increase in density and volumetric heat capacity caused by compaction. The thermal conductivity of the polyiminoborane powder was approximately three times lower, likely due to its higher porosity. The thermal diffusivity and conductivity of this product changed from 0.21 to 0.12 mm(2)/s and 0.068 to 0.23 W/m K (+/-10%), respectively, when its packing density was increased from 0.13 to 0.96 g/cm(3). Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.