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This paper investigates the contamination of the surface of various insulators suspended in the air at a certain altitude and connected in series. It is shown that salt-proof dishes attached to pin-type insulators are effective against contamination. Exposure tests of insulators on Okinawa have been made since 1985 and the results are as follows: (1) The higher the placement of the insulator, the heavier the contamination. (2) The amount of contamination on the downward surface of the suspended insulators at the lowest position was considerably smaller than that of the insulators at the other positions. (3) The extent of contamination of the pin-type insulators with the salt-proof dishes was far less in comparison to that of pin-type insulators without salt-proof dishes. (4). The maximum equivalent salt-deposit density was related closely to the small amount of precipitation during the observation period and the occurrence of high wind speed, which was higher than 10 m/s. [ABSTRACT FROM AUTHOR]