The tracking phenomenon for silicone rubber material under AC and DC voltages, with ammonium chloride as the contaminant, has been studied. The tracking phenomenon is explained in detail with a leakage current measurement. The tracking time depends on the conductivity and the flow rate of the contaminant. The surface condition of the aged specimens was characterized with contact angle measurements. The thermal and chemical stabilities of the material were identified with a methodical experimental study. Physicochemical analysis by wide-angle X-ray diffraction (WAXD), thermogravimetric differential thermal analysis (TG-DTA), and the differential scanning calorimetry (DSC) indicated that the mechanism of the tracking process is due to surface degradation. The tracking time is different for AC and DC voltages. (C) 2003 Wiley Periodicals, Inc.