This paper presents simulations of pollution and their effects on the electrical performance of cap-and-pin glass suspension insulators. The pollution is modeled by using a partially conductive epoxy resin with graphite powder. The effects of the pollution levels as well as their locations on the insulator surfaces are investigated for both simulation and experimental studies. The studies are conducted both for one suspension unit and a string of five insulators. The results show that at power frequency voltage, the reduction in the flashover voltage is about the same whether the upper surface or the bottom surface of the insulator is polluted. Moreover, the percentage reduction of a single unit is about the same as for a five unit string. At lightning impulse voltage, the insulator performance depends on the location of the pollution. Positive polarity impulse voltage is more critical than the negative polarity for the upper surface polluted case for one and the five unit string. For the bottom polluted case, negative polarity impulse voltage is more critical.