Abstract: A mounting arrangement for a pair of relatively high voltage, press-pack semiconductors positioned in spaced apart relationship on a support surface of a heat sink comprises an enclosure formed of electrically insulative material having a mounting surface mating with the heat sink support surface. The arrangement has a pair of spaced openings extending through the mounting surface, each of which is configured for relatively accurately positioning a respective one of the semiconductors. A groove circumscribes each opening with a third groove spaced from and circumscribing the pair of grooves on the mounting surface. An O-ring in each groove defines an area between the grooves isolated from environmental contaminants when the enclosure is clamped to the heat sink. An electrically conductive mounting plate is positioned in each opening and electrically insulated from the heat sink but electrically connected to a respective semiconductor.