Silicone rubber is widely used in the electrical engineering industry, especially for outdoor insulation, due to its good surface properties and the capability to recover hydrophobicity. The material used for this kind of application often is heavily filled high temperature vulcanizing silicone rubber. In harsh environments, tree-shape erosion patterns were observed on the rubber surface. The specific environmental conditions also led to a loss of the silicones hydrophobic characteristics. To further investigate outdoor performance of silicone rubber, resistance to erosion and tracking was tested according to the IEC587 standard at 4.5 kV. A large scatter in the results was found and attributed to the vague scope of the test method. For all materials tested, a layer-wise degradation was observed, which could be explained by the evaporation of ATH crystal water. The degradation product was found to be orthorombic mullite Al6Si2O13 formed at temperatures above 1200 °C during surface discharges.