A condensed floating monolayer of an amphiphilic benzothiazolium styryl dye containing a 1,10-dithia-18-crown-6 ether group has been found to complex with Ag+ and Hg2+ cations in acidic subphases. The efficiency of the processes was more than 2 orders of magnitude greater than expected for homogeneous solutions. This was attributed to the formation of L(2)M complexes (L = ligand; M = metal cation) at salt concentrations of 10(-7) to 10(-6) M. "Sandwich" complexes with composition L(n)M(n-1) (n > 2) may form at higher salt concentrations. Surface pressure versus area isotherms measured on Ag+-containing subphases showed changes for Ag+ concentrations in the range 10(-7) to 10(-4) M. However, BTC Langmuir-Blodgett films built up from acidic subphases containing either AgClO4 or Hg(CLO(4))(2) only exhibited hypsochromic shifts in their long wavelength absorption peaks for salt concentrations from 10(-7) to 10(-6) M.