We have studied selective chemical vapor deposition (CVD) of copper using (hfac)Cu(VTMS). Copper films were deposited selectively onto tungsten (W) and titanium nitride (TiN) in the presence of silicon dioxide (SiO2) surfaces in a warm-wall CVD reactor. Selective deposition was achieved by in-situ passivation of SiO2 surface hydroxyls by exposure to a silylating agent. These agents, such as dichloro-dimethylsilane and hexamethyldisilazane (HMDS), suppressed the nucleation rate of copper on SiO2 compared to metal surfaces. Control over loss of selectivity during CVD was achieved by maintaining the flow of the silylating agent during deposition. The passivation chemistry was studied by FTIR on a model Cab-O-Sil surface which showed that dichlorodimethylsilane and HMDS pre-dosing shielded the hydrogen-bonded surface hydroxyl groups making them inaccessible to the precursor molecule. An understanding of SiO2 surface modification and control over loss of selectivity during CVD is a key for successful selective copper deposition.