A detailed mineralogical investigation was made of 25 samples from nine deposits geographically encircling the Sudbury basin. This study has resulted in the identification and characterization of 13 platinum-group minerals, as well as in a description of their textures, mode of occurrence, mineral associations, and other details relevant to improving beneficiation.The platinum-group minerals found are froodite, insizwaite, kotulskite, merenskyite, mertieite II, michenerite, moncheite, niggliite, sperrylite, sudburyite, unnamed Pd(Te, Sb,Bi), unnamed Pd 8 Sb 3 , and unnamed Ag 4 Pd 3 Te 4 . Most of the opaque minerals in these samples were analyzed in situ for minor substitutions of the major metals by Pd, Pt, and Rh with the following results: Pd occurs in minor, but, in some cases, significant quantities in cobaltite, gersdorffite, insizwaite, maucherite, melonite, moncheite, nickeline, and niggliite; Pt occurs in cobaltite-gersdorffite, froodite, michenerite, and nickeline; Rh was found in cobaltite-gersdorffite and sperrylite.The South Range and Offset deposits are characterized by the presence of arsenides and antimony and by the scarcity of tin-bearing platinum-group minerals; in contrast, arsenides and antimonides are sparse and tin is present in platinum-group minerals in the North Range deposits. This difference is reflected in the platinum-group mineralogy; sperrylite is the principal platinum mineral in the former deposits, whereas moncheite, insizwaite, and niggliite are the principal platinum minerals in the North Range deposits. Other differences in the geochemistry between the South and North Range deposits are reflected in the most common palladium mineral, michenerite. South Range michenerite is usually antimonian- and platinum-free, whereas North Range michenerite is platinian and contains from nil to trace amounts of antimony.Much of the Pd and Pt, now found as discrete platinum-group minerals, is considered to have been fractionated into a Cu-rich liquid upon crystallization of monosulfide solid solution from an original immiscible sulfide-silicate magma. Crystallization of an intermediate solid solution from the Cu-rich liquid resulted in further enrichment of platinum-group elements in a Pd-Pt-Te-Bi-Sb-(As?)-rich liquid. The crystallization of the platinum-group minerals, on further cooling, was subsequently controlled by the phase relations in that type of system.