The process chemistry is the key to separation (1). It must allow separation of the platinum group metals from the base metals, where the major difference is in complex formation, and the separation of individual platinum group metals from each other, to a high degree of purity. The chloride system provides the most effective operating medium for platinum group metals and is widely used. The separation process chemistry considered here is therefore based on this system. The normal platinum group metals species encountered are shown in Table I. These species can be equated in weak chloride solutions and water, which is inhibited in stronger chloride media. Platinum group metal complexes are generally much more stable than the equivalent base metal complexes and this allows platinum group metals/base metals separation. Complexes containing the heavier donor atoms are more stable and the following overall order applies: