Two different phosphonic acid monolayer films for immobilization of bioactive molecules such as the protein BMP-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). The terminal functional groups on these monolayers were activated (carbonyldiimidazole for hydroxyl groups and N-hydroxysuccinimide for carboxyl groups) to bind amine-containing molecules. The reactivity of the surfaces was investigated using trifluoroethylamine hydrochloride and BMP-2. Each step of the surface modification procedure was characterized by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry.