The charge transfer (CT) from montmorillonite K10 (iron-containing clay) was studied by monitoring the reaction of halides (Cl-, Br-, and I-) with the intercalated photosensitizer, 10-methylacridinium (AcH+). The electrodes, which were fabricated by the AcH+-intercalated clays (denoted clay/AcH+), showed no current responses to bromide and chloride. However, there was a significant current response corresponding to the reduction of iodine and this response was enhanced by light irradiation. These differences in the electrochemical behavior suggested the electron transfer on the clay/AcH+ surface was regulated by the electrochemically active iron species in the clay. The lifetime of AcH+ is ca. 36 ns, whereas for AcHclay+, it is as fast as the instrumental response (<10 ns), indicating the clay can efficiently quench the excited acridinium, AcH+* Moreover, a long-lived species, attributed to I-2(-.), was recorded in KI solutions (pH 4) containing either AcH+ or clay/AcH+. In contrast, the transient species, presumed to be Br-2(-.), was only observed in the solutions containing free AcH+. Both electrochemical and laser spectroscopic results indicated that the electron transfer between AcH+* and I- is mediated by the iron species in montmorillonite K10.