In this study, the effects of thermally conductive filler type (alpha-Al2O3, SiC), volume fraction of the filler, and filler particle size distribution on the thermal conductivity and viscosity of room-temperature-vulcanized (RTV) silicone rubber and silicone grease were investigated. We were interested to find that silicone grease (or the RTV silicone rubber) had a maximum thermal conductivity (similar to1.48 W/mK) and a minimum viscosity (similar to3.4 X 10(4) mPa s), with a definite total volume fraction of the filler (0.55) when the distribution of filler sizes (the number ratio of two different particles sizes, i.e., 0.8 and 6 mum) was 600-700. We were able to increase the thermal conductivity of the RTV silicone rubber and silicone grease beyond 2 W/mK by increasing the total volume fraction of the filler with adequate filler size distributions. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2397-2399, 2003.