The oxygen reaction at the interface LaF₃(CeF ₃)|electrode, O₂(g) is studied in air on Pt and C electrodes and electrodes modified by supported oxides of Co, Sr, and/or La on C, Si, or Pt substrates. The experimental methods used are voltammetry with a linear potential change and stripping voltammetry and cyclic voltammetry. The voltammetric response is dependent on the nature of solid electrolyte and electrode, the balance between the substrate and active agent, and on the conditions of thermal treatment of electrodes. Mobile oxygen forms are discovered at interfaces CeF₃|C (Co_xO_y), O ₂ ([C] : [Co] = 1 : 10, 1 : 20); CeF₃|C (Co _xO_y), O₂ ([C] : [Co] = 1: 10); and CeF ₃|Pt (Co_xO_y), O₂ ([Pt] : [Co] = 1 : 40). The peak current I_pa in the anodic curves increases with extending the limit of cathodic potential scan E_rc. The peak current I_pc in the cathodic curves increases with extending the limit of anodic potential scan E_ra.