To avoid potential misapplication of effective thermal conductivity models, materials that may be described as `porous' should be divided into two classes; `internal porosity' materials which have bubbles/pores suspended within a continuous condensed phase (e.g. sponges, foams, honeycombs), and `external porosity' materials which include granular/ particulate materials. It is proposed that the effective thermal conductivity region bounded by the Hashin-Shtrikman bounds may be divided into internal porosity and external porosity regions by the Effective Medium Theory (EMT) equation. The use of the Hashin-Shtrikman and EMT equations as porosity bounds was supported by experimental data from the literature. (c) 2005 Elsevier Ltd. All rights reserved.