Surface reaction mechanisms in the chemical vapor deposition of (Ba,Sr)TiO3 [BST] films were studied by investigating the effects of O2 gas and source supply ratios on the characteristics of atomic incorporation rates. The atomic incorporation rates of Ba, Sr, and Ti increased with increasing incident flux of each source material, and then the values of the atomic incorporation rates became saturated. The saturated values increased monotonously with increasing O2 gas flow rate, in a range where atomic incorporation reactions might be controlled by the kinetics on the film surface (kinetically limited). Accordingly, O2 gas may effect the behavior of film precursors on BST film growth surfaces. From this, we assumed a CVD model, where the precursors are transported onto the film surface and adsorbed on adsorptive sites, and where the O2 gas has an effect on the formation of the adsorptive sites. With this model, atomic incorporation rates and overall sticking coefficients for the CVD of BST films were numerically simulated, and were in good agreement with experimental results for several O2 flow rates and source supply ratios.