The adsorption of oxacarbocyanines on silver halide microcrystals was studied. A point of discontinuity was observed in the adsorption isotherm of 5, 5'-dichloro-9-ethyloxacarbocyanine on AgBr microcrystals. At this point, the adsorption state changed from monomer to J-aggregate. The equilibrium concentration decreased with increasing amount of the dye adsorbed in J-aggregate; this suggests the existence of a large interaction between adsorbed molecules. Lowering the equilibrium concentration resulted in decomposition and desorption of J-aggregate, while the dye molecule adsorbed in monomer state was not desorbed. Formation of J-aggregate is facilitated by substituting 5-chlorine to 5-phenyl of oxacarbocyanine, on the surface of silver halide containing iodine, or by lowering the methanol content of solution. The occurrence of an intermolecular interaction between different kinds of oxacarbocyanines was verified.