The metalorganic precursor chemistry was studied on Pt(111) surfaces in a 16O2 and 18O2 backgrounds. Using temperature programmed desorption (TPD) and static secondary ion mass spectrometry (SIMS). The precursor chemistry of Sr(thd)2 was found to be different on oxide covered Pt(111) surface as compared to the clean Pt(111) surface. In an oxygen ambient, TPD showed at least four different reaction processes which involved the removal of carbon from the precursor ligands on oxide covered Pt(111). In two of these, gas phase oxygen was incorporated into the oxidative products. In contrast, one carbon removing reaction was observed on the clean Pt(111) surface. Isotopic labeling experiments have also been carried out to understand the film-formation reactions in the metalorganic chemical vapor deposition of (Ba,Sr)TiO3 (BST) films. Time-of-flight SIMS and nuclear reaction analysis reveal that the oxygen in the BST films originates from both the gas phase oxidants (18O) and the precursor ligands (16O). The ligand substitution by gas phase O2 plays a more prominent role in the film formation at lower temperatures. On the other hand, the reactive oxygen radicals produced by microwave plasma involved more in breaking the O–C bonds than substituting the precursor ligands for the film formation. Use of the 50% 18O2–50% N2 16O2 mixture results in a reduction of 18O incorporation in the BST films, indicative of the direct involvement of N2O in the film-formation reactions. The mechanistic studies are essential for understanding the new BST precursors used in this study, and provide useful information to correlate the film microstructure, step coverage, and dielectric properties with the precursor properties. © 2000 American Institute of Physics.