The amphiphilic benzodithia-15-crown-5 styryl dye forms relatively stable insoluble monolayers on distilled water (collapse pressure is 34.5 mN m - 1 ) and alkali metal cations (35.6-36.7 mN m - 1 ) or heavy metal cations (24.2 mN m - 1 for Ag + ) in the aqueous subphase. Three types of the dye monolayer structures depending on the subphase nature (water, alkali metal or heavy metal cations) have been distinguished from surface pressure-molecular area and surface potential-molecular area isotherms, as well as observed by Brewster angle microscopy (BAM). The ionoselective and photosensitive properties of the amphiphilic dye in monolayers have been studied in the presence of K + , Na + , Hg 2 + and Ag + cations in the aqueous subphases.