The effective thermal conductivity of Cu-Fe and Sn-Al miscibility gap alloys over a range of temperatures and volume fractions was determined using the Lattice Monte Carlo method. The Cu-Fe system was found to have an effective conductivity predictable by the Maxwell-Eucken model. The Sn-Al system was not consistent with any empirical model analysed. The microstructures of physical samples were approximated using a random growth algorithm calibrated to electron or optical microscope images. Charts of effective conductivity against temperature for a number of volume fractions are presented for the two alloys. It was determined that the Cu-Fe alloy would benefit from an interstice type microstructure and the Sn-Al would be more efficient with a hard spheres type microstructure. More general conclusions are drawn about the efficiency of the two observed microstructures. (C) 2014 Elsevier Ltd. All rights reserved.