The decomposition of bioactive plasma sprayed apatite layers and the weakness of their interface with the metallic substrate limit the longevity of orthopaedic prostheses. Analysis of the coating and of the metal-apatite interface using EDS, XPS, and IR techniques indicates alterations of the apatite composition which can be related to several chemical reactions occurring either in the plasma or on the surface of the implant. EDS shows a calcium-rich layer on the apatite side of the interface whereas after dissolution of the apatite, XPS indicates that phosphorus atoms are incorporated in the metal surface. Depending on the rate of decomposition, calcium oxide may possibly form and weaken the apatite-metal interface. Fluorohydroxyapatite coating have proven to decompose less and differently and to be more effective than hydroxyapatite coatings.