A long-chain merocyanine derivative with the chlorine substituent at the 5-position of the benzothiazol moiety has been studied for formation of the stable J-aggregates in the mixed monolayers and their LB films with Cd perfluorooctadecanoate (PFC18) and octadecane (OD) in comparison with the one without the substituent, which forms the H-aggregates in the mixed films with Cd arachidate and OD. In addition to the monolayer pi-A isotherms, visible absorption spectra, the grazing angle in-plane X-ray diffraction and the AFM observation indicate that PFC18 promotes J-aggregation of the dye due to the phase separation in the mixed films. (c) 2005 Elsevier B.V. All rights reserved.