A filler system for polymers is disclosed which provides high voltage insulation which is resistant to tracking. The filler system utilizes a combination of alumina trihydrate and a chemically treated silica filler. The silica-treated filler results from the exposure of an inorganic silicon-containing filler having a specific surface area of at least 40 square meters per gram to one or more silanes. Preferred silanes are substituted silanes having the formula R.sub.n Si X.sub.4.sub.-n where n is 1, 2 or 3, R is an organic radical bonded to the silicon atom by a Si--C bond and X is a radical bound to the silicon atom via an atom other than a carbon atom.