The modes of occurrence of the potentially hazardous elements in coal will be of significance in any attempt to reduce their mobilization due to coal combustion. Antimony and selenium may be present in solid solution in pyrite, as minute accessory sulfides dispersed throughout the organic matrix, or in organic association. Because of these modes of occurrence it is anticipated that less than 50% of these elements will be routinely removed by conventional coal cleaning procedures. Arsenic and mercury occur primarily in late-stage coarse-grained pyrite therefore physical coal cleaning procedures should be successful in removing substantial proportions of these elements. Cadmium occurs in sphalerite and lead in galena. Both of these minerals exhibit a wide range of particle sizes and textural relations. Depending on the particle size and textural relations, physical coal cleaning may remove as little as 25% of these elements or as much as 75%. Manganese in bituminous coal occurs in carbonates, especially siderite. Physical coal cleaning should remove a substantial proportion of this element. More information is needed to elucidate the modes of occurrence of beryllium, chromium, cobalt, and nickel.