The novel amphiphile 5-(octadecyloxy)-2-(2-thiazolylazo)phenol (TARC18) formed a stable condensed type monolayer at the air/water interface. When transition metal ions such as Fe(II), Co(II), Ni(II), Cu(II), zn(II), and Cd(II) were present in the aqueous subphase, 1:1 complexes of TARC18 with the above metal ions were formed in the monolayers at the air/water interface. The formation of these complexes was verified by measurements of the pi-A isotherms and the absorption spectra of the monolayers at the air/water interface in situ. The TARC18 monolayer showed strong ability to incorporate the Cu(II) ion in the subphase, resulting in a larger Limiting area and lower collapse pressure of the monolayer on the subphase containing Cu(LI) ion in comparison with those on the subphase containing the other metal ions. Moreover, the Cu(II) ion was predominantly bound to the TARC18 monolayer when all the above ions coexisted in the subphase.