A review of MOCVD BST thin films will be presented, focusing on precursor chemistry, film-formation mechanisms, and relationship between film processing, microstructure and dielectric properties. In particular, the precursor chemistry and film-formation r eactions have been studied using isotopic labeling experiments (@super 18@O@sub 2@). The precursor chemistry was found to strongly depend on substrate materials. In an oxygen ambient, at least four different reaction processes involved the removal of carb on from the precursor ligands on oxide covered Pt(111). Time-of-flight secondary ion mass spectrometry reveals both M@super 18@O and M@super 16@O (M= Ba, Sr, Ti) in the BST films, indicating that the oxygen in the BST films originates from both the gas ph ase oxidants (@super 18@O), and the precursor ligands (@super 16@O). The ligand substitution by gas phase O@sub 2@ plays a more prominent role in the film-formation at lower temperatures. On the other hand, the reactive oxygen radicals produced by micr owave plasma involved more in breaking the O-C bonds than substituting the precursor ligands for the film formation, resulting in larger surface roughness. Use of the 50%@super 18@O@sub 2@-50%N@sub 2@@super 16@O mixture results in a reduction of @super 18@O incorporation in the BST films, indicative of the direct involvement of N@sub 2@O in the film-formation reactions. The mechanistic studies are essential for understanding the BST precursor properties, and provide useful information to correlate the fil m m icrostructure, step coverage, and dielectric properties with the precursor properties. In addition, the study shows that precursor reactivities strongly affect the step coverage on the 3D structure, but little effect on the microstructure, surface morphology, and dielectric properties of the stoichiometric BST films. These properties strongly depend on the film composition, substrate material, and growth temperature.