The chemistry and morphology of aluminide coatings formed on platinum and platinum-coated IN-738 have been studied. Most of the aluminide coatings evaluated were applied using the pack cementation process. For aluminized platinum a series of intermetallic Pt-Al compounds form. The stoichiometries of these compounds are essentially in agreement with those that would be predicted based upon phase diagram considerations. For aluminized, platinum-coated IN-738, the coating morphology and chemistry are highly dependent upon the thickness of the platinum layer. A relatively thick platinum layer (∼25 microns) confines the initial reaction for the aluminizing conditions used so that refractory metal elements from the substrate are excluded from the outer regions of the coating. Thinner platinum layers only partially confine the reaction and do not exclude the refractory metals from the coating. Microstructures that develop are related to the appropriate phase diagrams.