Redox isomerism, that is, the change of a metal cation valence state in organic complexes, can find promising applications in multistable molecular switches for various molecular electronic devices. However, despite a large number of studies devoted to such processes in organic complexes of multivalent lanthanides, redox-isomeric transformations were never observed for europium. In the present work, we demonstrate the unique case of redox isomerization of Eu(III)/Eu(II) complexes on the example of Eu double-decker octa-n-butoxyph-thalocyaninate (Eu[BuO)(8)Pc](2)) under ambient conditions (air and room temperature). It is shown that assumption of the face-on orientation on the aqueous subphase surface, in which two of each phthalocyanine decks in Eu[(BuO)(8)Pc](2) are located in different media (air and water), leads to the intramolecular electron transfer that results in the formation of a divalent Eu(II) cation in the complex. Lateral compression of the thus-formed monolayer results in the reorientation of bisphthalocyaninate to the edge-on state, in which the ligands can be considered identical, and occurrence of the reverse redox-isomeric transformation into the complex with a trivalent Eu cation. Both redox-isomeric states were directly observed by X-ray absorption near-edge structure spectroscopy in ultrathin films formed under different conditions.