A series of bare ZrO2 materials composed either of monoclinic or tetragonal phase with the size of crystallites varying between 3.7 and 43.4 nm were prepared by precipitation, hydrothermal or thermal calcination methods. The samples were characterized by XRD, BET, CO-TPR, NH3-TPD, electrical conductivity measurements and operando UV-vis spectroscopy. Density functional theory calculations provided molecular insights into the kind of active site and individual steps of propane dehydrogenation to propene and hydrogen. Regardless of the phase composition, two Zr cations located at an oxygen vacancy, i.e. coordinatively unsaturated Zr cations (Zr-cus sites) having, nevertheless, different chemical environment in tetragonal and monoclinic ZrO2, were concluded to be responsible for homolytic breaking of C-H bonds in propane. Monoclinic ZrO2 showed, however, higher rate of propene formation and higher propene selectivity than ZrO2 stabilized in the tetragonal phase. Both the activity and the selectivity to propene increased with a decrease in the size of crystallites. The effects of phase composition and crystallite size on the PDH performance were related to the ability of ZrO2 to release lattice oxygen upon reductive catalyst treatment and thus to create Zr-cus sites. The knowledge derived can be used for further optimizing PDH performance of ZrO2-based catalysts and also extended to other non-reducible metal oxides. (C) 2019 Elsevier Inc. All rights reserved.