Mixed monolayers of 5‐(4‐N‐decylpyridinium)‐10,15,20‐tri‐p‐tolyl porphyrin (PyP) and arachidic acid (AA) were prepared on a water surface as a precursor for organized arrays of photodiodes. By compression and expansion of the films, PyP self‐assembled to form nearly circular domains with their size dependent on the molar ratio of PyP and AA. These structures were successfully incorporated into Schottky‐type cells, and their photoelectric properties were studied on the domain level by illumination of a focused light with a diameter of 20 &mgr;m. The short‐circuit photocurrents showed large fluctuations depending on the position of illumination, which reveals that the PyP domains function as photodiode arrays.