The Merensky Reef at the Western Platinum mine differs in many respects from the reef at its type locality at Rustenburg, 30 km to the west.The reef averages 4 m thick and consists of major porphyritic melanorite and minor pegmatitic melanorite enclosing two thin chromitite bands. Platinum-group mineralization occurs mainly in the vicinity of the upper chromitite seam, whereas at Rustenburg, where the reef is very thin and consists of pegmatitic pyroxenite, the mineralization occurs near both the chromitite seams. It is thought that a facies change occurs in the reef between the two localities.The proportions of the platinum-group minerals are different from those at all the previously recorded localities. Sperrylite is the most abundant mineral, constituting more than 50 percent of the platinum-group minerals. Braggite and cooperite, together with moncheite and minor kotulskite, are present in approximately equal quantities, each assemblage constituting about 20 percent of the platinum-group minerals. Minor amounts of platinum-iron alloy, gold, stibiopalladinite, and laurite are also present.The Pd, Ru, Rh, and Ir contents of the minerals are inconsistent with the contents of these elements in the reef, and it is suggested that some Pd is in solid solution in pentlandite and that Ru, Rh, and Ir are associated with pyrrhotite and pentlandite.The lateral distribution of platinum-group elements in the reef at Marikana has a very irregular trend.It is proposed that the Merensky Reef was formed by gravity concentration but that it was extensively modified by subsequent hydrothermal alteration, which gave rise to variations in the mineralogy and distribution of the platinum-group elements at different localities along the strike of the reef.