Oxygen adsorption on YBa₂Cu3O_6.9 ceramics at T = 250-400 K was studied by thermal desorption (TD) mass spectrometry. It was established that, depending on the adsorption temperature (T_a), oxygen occurs on the substrate surface either in two forms (γ1 and γ2, for T_a < 350 K) or in a single form (γ2, for T_a ≥ 350 K). In the TD spectrum, γ1 adsorbed oxygen species are manifested by a peak with a maximum at T_max = 330-340 K. When the adsorption temperature increases from 290 to 400 K, the TD peak of γ2 oxygen species shifts from T_max ≈ 380 K to 440 K, which is accompanied by a decrease in the temperature "boundary" for the desorption of structural oxygen. In addition, γ2 oxygen species are capable of participating in isotope exchange with the oxygen of the ceramics. For both forms of adsorbed oxygen, TD most probably proceeds by the associative mechanism with an activation energy of 0.63 ± 0.08 eV (γ1) and 1.13 ± 0.02 eV (γ2). A model qualitatively explaining oxygen sorption in ceramics is proposed, according to which γ2 species are formed in the initial adsorption stage.