Current understanding of the behaviour of plasma discharges within packed bed reactors (PBRs) is very poor, and the effects of many of the parameters that can be varied are still unknown. This article investigates the effects of particle size (180 mu m-2000 mu m) of two different commonly used packing materials (Al2O3 and BaTiO3) on the conversion of CO2 in PBRs. The reactor behaviour is observed through determination of product gas composition and plasma power consumption in order to determine CO2 conversion and reactor efficiency. Electrical characterisation techniques are used to determine reactor burning voltage, and capacitances. These capacitances are subsequently used to quantify the occurrence of reactor partial discharging over a range of different operating conditions. The results indicate that smaller particles (down to 180 mu m) can significantly increase CO2 conversion by up to 70%, provided that the voltage applied is sufficiently high to generate a discharge in the void spaces of the packing material. However, with decreasing particle size, the reactor burning voltage is found to increase rapidly, as well as the tendency of the reactor towards partial discharging. (C) 2016 The Authors. Published by Elsevier B.V.