Design of high performance materials for thermal insulation or heat sinking requires understanding of the mechanisms controlling heat flow. The effect of grain size on the effective thermal conductivity of porous alumina ceramics has been investigated. After uniaxial pressing of a fine a-alumina powder, control of firing temperature and duration yielded ceramics with variations in grain size. Values of thermal conductivity corresponding to equivalent 100% dense ceramics were determined from laser flash measurements using Landauer's relation to take porosity into account. These exhibited a strong decrease from 33 W m(-1) K-1, for grain sizes larger than 2 mu m, down to 8 W m(-1) K-1 for an average grain size of 0.25 mu m. A method to separate the contributions of localized thermal resistance (Kapitza resistance) at grain boundaries and grain conductivity for each sample is presented. The larger grain size samples (>0.5 mu m) yield values for the effective grain boundary thermal resistance in the range 0.6-0.8 x 10(-8) m(2) K W-1 and a grain conductivity close to 35 W m(-1) K. For smaller grain samples (<0.5 mu m), the grain boundary thermal resistance increases to 2 x 10(-8) m(2) K W-1 while the grain conductivity reduces to 23 W m(-1) K-1. Thus grain size controls both the number of grain boundaries in the heat path and the grain conductivity itself. (C) 2018 Elsevier Ltd. All rights reserved.