Transient plane source technique was used for the simultaneous measurement of thermal conductivity and thermal diffisivity of three-phase styrene butadiene rubber composites. Two series of styrene butadiene rubber composites were studied, having natural rubber as a variable filler in both the composites along with 10 phr of silica and clay, respectively. The measurements were done at room temperature and normal pressure. The experimental results show that there is a small variation in the thermal conductivity of both the composites with the filler (NR) fraction. It is interesting to note that the thermal conductivity shows a sharp decrease at 10 phr filler loading and then increases. The comparative study of these composites shows that the conductivity as well as the diffusivity of the silica reinforced composites is larger than that of the clay composites. The thermal conductivity of the filler NR has been evaluated using the Agari model. It has also been found that the composite with 40 phr of NR has the maximum thermal conductivity. (C) 2000 John Wiley & Sons, Inc.