We report monitoring and analysis of the contamination overlayer on the surface of different SrTiO3 (STO) substrates by in situ spectroscopic ellipsometry (SE) and ex situ X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Substrates of STO with different terminations, random and TiO2 terminated, were heated from room temperature up to 720 degrees C under oxygen pressure in UHV chamber similar to conditions commonly used for epitaxial growth of perovskite oxides. Contamination layer on the substrate was modeled as an equivalent dielectric overlayer with a thickness of 2 nm at room temperature which decreases progressively during the heating up to reach its minimum (around 1 unit cell) at the temperature around 550 degrees C. After exposition to air, surface recovers a contamination layer on both types of substrates (with random termination and TiO2 termination). XPS analysis confirmed that water and carbon dioxide as adventitious carbon species present in air are chemically adsorbed on the STO surface, providing evidence of desorption process which persists until 550 degrees C. This condition is an important issue in order to obtain clean controlled interface between STO and deposited film for low temperature growth as for instance atomic layer deposition and integration of STO buffer layer on silicon. In situ SE commonly present in thin layer deposition systems is a powerful tool to monitor in situ surface contamination and decontamination temperature as it can be performed in situ even in operando. (C) 2016 Elsevier B.V. All rights reserved.