Langmuir-Blodgett films have been built-up from a novel benzothiazolium steryl dye containing a 1,10-dithia-18-crown-6 ether group and their complex formation with Ag+ cations studied by absorption spectroscopy. Films of a few molecular layers thickness were found to exhibit two stages of complex formation, with stability constants of about 4 x 10(6) and 4 x 10(5) M(-1). Both these values exceed the constant expected for a homogeneous aqueous solution (2 x 10(4) M(-1)). The complex formation was also found to be controlled by diffusion. Furthermore, the reaction was partly reversible.