Alkali-treated orthopaedic titanium surfaces have earlier shown to induce apatite deposition. A subsequent heat treatment under air improved the adhesion of the sodium titanate layer but decreased the rate of apatite deposition. Furthermore, insufficient attention was paid to the sensitivity of titanium substrates to oxidation and nitriding during heat treatment under air. Therefore, in the present study, alkali-treated titanium samples were heat-treated under air, argon flow or vacuum. The microstructure and composition of their surfaces were characterized to clarify what mechanism is responsible for inhibiting in vitro calcium phosphate deposition after heat treatment. All heat treatments under various atmospheres turned out to be detrimental for apatite deposition. They led to the thermal decomposition of the dense sodium titanate basis near the interface with the titanium substrate. Depending on the atmosphere, several forms of TiyOz were formed and Na2O was sublimated. Consequently, less exchangeable sodium ions remained available. This pointed to the importance of the ion exchange capacity of the sodium titanate layer for in vitro bioactivity. (C) 2009 Elsevier B.V. All rights reserved.