In this work, a previously undescribed phenomenon of orientation-induced redox isomerism in a Langmuir monolayer is revealed in the case of cerium bis-[tetra-(15-crown-5)-phthalocyaninate] – (Ce[(15C5)4Pc]2). It was established that intramolecular electron transfer (IET) from the electronic system of phthalocyanine to the 4f-orbital of cerium atom occurs upon spreading of a (Ce[(15C5)4Pc]2) chloroform solution onto the air-water interface (3D2D IET). This process is related to the transformation of Ce4+ cation in the solution into Ce3+ in the monolayer. It was also found that reversible Ce3+Ce4+ IETs occur upon compression (1→2) and expansion (2→1) of monolayer (2D12D2 IET,  - surface pressure). The mechanism of genuine redox isomerism was confirmed by the results of in-situ UV-Vis spectral measurements performed on monolayers and Langmuir-Blodgett films, AFM, and XPS studies of Langmuir-Blodgett films transferred at different surface pressures. The understanding of this reversible IET mechanism is especially important due to possible applications of such redox-isomeric systems in the development of nanoscale multibit information storage devices.