The phospholipid topology of the outer membrane of intact rat liver mitochondria and derived outer membrane vesicles was investigated by determining the accessible pool of the various phospholipid classes towards phospholipase A2, a phosphatidylcholine-specific transfer protein and by chemical labeling using trinitrobenzene sulfonic acid. The outer membrane vesicles are sealed and have a right-side-out topology with a proposed localization of 55%, 77%, 100%, and at least 30% of the phosphatidylcholine, phosphatidylethanolamine, cardiolipin, and phosphatidylinositol plus phosphatidylserine in the outer leaflet, respectively. The outer membrane in intact mitochondria appears to have a similar phospholipid distribution.