(Ba0.5Sr0.5)TiO3 (BST) thin films were deposited on MgO, Si, SiO2 and Ir surfaces by ion: beam sputter deposition in oxygen at 700 degrees C. In situ spectroscopic ellipsometry (SE) has been used to investigate the evolution of the BST films on different surfaces-during both deposition and postannealing processes; First, the optical constants of the BST films in the photon energy range of 1.5-4.5 eV were determined by SE analysis on crystallized BST films;deposited on MgO single crystal substrates, The interfaces in BST/Si and BST/SiO2/Si structure were examined by SE and Auger electron spectroscopy depth profiles. Subcutaneous oxidation in the BST/Ir structure was:: observed by in situ SE during both ion beam sputter deposition and postdeposition annealing in oxygen at 700 degrees C. A study of the thermal stability of the Ir/TiN/SiO2/Si structure in oxygen at 700 degrees C was carried out using in situ SE. The oxidation of Ir was confirmed by x-ray diffraction. The surface composition and morphology evolution after oxidation were investigated by time of flight mass spectroscopy of recoiled ions (TOF-MSRI) and atomic force microscopy. It has been found that Ti from the underlying TiN barrier layer diffused through-the Ir layer onto the surface and thereupon became oxidized. It was also shown that the surface roughness increases with increasing oxidation time. The implications of the instability of Ir/TiN/SiO2/Si structure on the performance of capacitor; devices based on this substrate are discussed. It has been, shown that a combination of in situ SE and TOF-MSRI provides a powerful methodology for in situ monitoring of complex oxide film growth and postannealing processes.