Ion implantation of calcium has been proposed previously as a route to bioactive titanium surfaces and has been shown to stimulate promising cell and tissue responses. While the precise reasons for this behaviour remain poorly understood, it is clear that the nature of the Ca implanted surface changes rapidly on exposure to body fluids. In order to understand the processes taking place more clearly, the current work examined the simple interaction of Ca implanted Ti with water. The surface chemistry and compositional changes within the sub-surface region of the modified Ti were examined. On immersion in water, the concentration of implanted Ca ions was found to decrease both at the surface and throughout the implanted region. At the same time, the sub-surface oxygen concentration was found to increase dramatically. Although Ca implantation into Ti results in a thicker oxide layer at the surface, it appears that this layer no longer affords the underlying Ti the same protection from further oxidation provided by the native oxide. By examining samples implanted with O, Ti or Ar it was possible to conclude that this was specific to Ca implantation and not a result of the ion implantation process itself. (c) 2006 Elsevier B.V. All rights reserved.