The thermal conductivity of various copper matte and fayalite slag was measured using laser flash analysis, a non-steady state measurement method. Industrial matte and slag samples were taken in such a way that their composition represented typical process conditions. Thermal conductivities for solid copper matte (average 64% Cu) were found to be from 1.2 W m−1 K−1 at 300°C to 2.1 W m−1 K−1 at 900°C. Because arsenic is one of the most important impurities in copper matte, its effect on thermal conductivity was investigated with As-doped matte samples up to 0.59% As. The results showed substantially lower thermal conductivity, between 0.5 W m−1 K−1 and 1.3 W m−1 K−1 at 300–900°C with low As matte, behavior that is analogous to that of a semiconductor. The data obtained showed that the thermal conductivity of copper matte increased linearly with temperature, but the gradient was small. The thermal conductivity of slags was found to be between 1.6 W m−1 K−1 and 1.9 W m−1 K−1, values that are consistent with earlier studies.