The in-vitro behavior of hydroxyapatite (HAp) coatings depends on the thermal history of the coating after thermal spraying. As-sprayed amorphous coatings degrade in Ringer's solution. Heat treatment at 800 °C for 2 hours produces a crystalline coating which upon immersion in physiological solution displays a greater stability. Coatings are characterized for surface morphology, composition, crystallinity, roughness, and weight loss. The as-sprayed coating shows changes in the coating morphology with immersion time. Individual lamellae crack and separate from the coating. Those lamellae still intact show evidence of dissolution. After a period of 8 weeks small HAp spheres cover the surface of the as-sprayed and heat treated coatings. These morphological changes are expected to influence the rate of bone bonding. Therefore post-treatment of HAp coatings is seen as a technique to alter or control implant-tissue interactions.