Influence of an ex situ annealing temperature and atmosphere on chemical composition and structural and superconducting properties of MgB2 thin films deposited by vacuum evaporation has been investigated. The annealing has been done in Ar, N2 and Ar + 5%H-2 atmospheres at pressure of 700 Pa and temperature varying from 700 to 800 degrees C. It has been shown that annealing in Ar and N-2 atmosphere at 700-800 degrees C produces relatively thick MgO layer on the surface of the films, while creation of such layer is highly reduced if the annealing is done in reducing Ar + 5%H-2 atmosphere. The XPS and XRD results suggest that the MgO layer prevents out-diffusion of Mg from the film during the annealing, what assures better stoichiometry of the films as well as creation of larger MgB2 grains. The films with the highest amount of MgO on the surface, annealed in nitrogen atmosphere, thus paradoxically exhibited the highest critical temperature of Tc0 =34.8 K with very sharp transition width of 0.1 K. (C) 2014 Elsevier B.V. All rights reserved.