Abstract: The aggregation behavior of 3.3'-dihydroxy ethyl 5.5'-dimethoxy 9 ethylthiacarbocyanine chloride (dye 798) in CH₃OH, H₂O solution and its adsorbed state on AgBr surface was studied by the absorption and reflection spectra and by the adsorption isotherms at 25℃. NO J-absorption,but only dimer-monomer equilibrium was observed form the absorption spectra of aqueous solutions of the dye in the experimental concentration range, the dimerzation constant K-O (298 K),ΔrG-Om(298 K),ΔrH-Om(298 K)and ΔrS-Om(298 K) were found spectrophotometrically to be 1.28×10^-5, 27.9 kJ/mol, 35.2kJ/mol and 24.5 J/(K·mol), respectively. The adsorption isotherm obtained at 25℃ indicated that the adsorption of dye 798 on the surface of AgBr was of multilayer type with a saturated capacity of 0.73 μmol/g(AgBr)(expressed as monomer units of the dye )in the first adsorbed layer. The surface reflection spectra showed that after the J band had reached critical intensity(λ_Ja=640nm),the further addition of dye only caused H-aggregate and monomer formation. This indicated that only the dye molecules in the first adsorbed layer existed as J-aggregates,the adsorption states in subsequent dye layers were different. The J-aggregates of cyanine dye play an important role in spectral sensitization to photographic emulsion, especially in ultra sensitization. The study of aggregation behavior has been an important aspect of spectral sensitization studies. The aggregation behavior of photographic sensitizing dye 798 used in monochrome in solution and the results of studying the adsorbed state on the surface of silver bromide and its aggregation thermodynamic properties were reported. All above will play an important role in using dye reasonably in productions and further understanding sensitizing mechanism.