The photoincrement of admittance (photoadmittance) of merocyanine‐arachidic acid mixed Langmuir–Blodgett films sandwiched between aluminium electrodes has been measured for frequencies &ohgr;/(2&pgr;)<1 Hz at room temperature using the monochromatized visible incident. The number of monolayers of the films was 3–19. The photoadmittance is ascribed to photoelectrons from the dye chromophores. The light‐intensity dependence obeys a square‐root law for thicker films, approaching a linear law as the film thickness decreases. The frequency dependence is found to originate from the dispersive component proportional to &ohgr;1−s, which predominates over the d.c. component for thicker films. The value ofs≊0.5 is evaluated with a critical length of about 30 nm as characterizing the crossover between the thin‐film and the thick‐film cases.