Formation of J-aggregates of the amphiphilic merocyanine 3-octadecyl-2-[3,3-dicyano-allylidenlbenzoxazolin (S118) in monolayers at the air-water interface has been investigated in the absence and presence of the additives (coaggregates, CA) octadecane, methylstearate, octadecanoic acid, and octadecanol, respectively, in the molar ratio S118/CA = 1: 1. The monolayers have been characterized by surface pressure-area and surface potential-area isotherms, and the morphology was studied by Brewster angle microscopy. Chromophore association was investigated by reflection spectroscopy. In the presence of the coaggregate, monolayer stability is increased and the morphology drastically changed, although J- aggregates are formed in all monolayers. Efficient energy transfer in monolayers at the air-water interface from excited S118 to an amphiphilic merocyanine incorporated in the monolayer as acceptor was detected by fluorescence spectroscopy.