The thermal conductivity of MOF-5 single crystals is measured over a wide temperature range between 6 K and 300 K, using the longitudinal, steady-state heat flow method. Between 6 K and 20 K, the thermal conductivity increases with the increase in temperature and exhibits a peak near 20 K. This peak results from the crossover between the decreasing mean free path and the increasing phonon specific heat with the increasing temperature. From 20 K to 100 K, the thermal conductivity decreases rapidly with increasing temperature. Above 100 K, the thermal conductivity is nearly temperature independent, and its value at 300 K is 0.32 W/m K, a rather low value for crystals. The mean free path analysis shows that at high temperature, the phonon mean free path is minimized to the cage size due to the porous, flexible structure of MOF-5. (c) 2006 Elsevier Ltd. All rights reserved.