Two different phosphonic acid monolayer films for immobilization of bioactive molecules such as the proteinBMP-2 on titanium surfaces have been prepared. Monolayers of (11-hydroxyundecyl)phosphonic acid and (12-carboxydodecyl)phosphonic acid molecules were produced by a simple dipping process (the T-BAG method). Theterminal functional groups on these monolayers were activated (carbonyldiimidazole for hydroxyl groups andN-hydroxysuccinimide for carboxyl groups) to bind amine-containing molecules. The reactivity of the surfaces wasinvestigated using trifluoroethylamine hydrochloride and BMP-2. Each step of the surface modification procedure wascharacterized by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry.