Apparently reversible and dissociation of small, metastable primary J- aggregate particles and dimeric particles of a phanchromatic carbocyanine dye I in moderatly dilute aqueous methanol solutions containing electrolytes (Kcl and Na2SO4) have been studied spectrophotometrically. Association numbers, n, for the particles were estimated from mass action relation between the concentration of free and aggregated dye molecules. Particle sizes, were also estimated from spectral absorption line widths. The thermodynamic properties like equilibrium
constant and free energy were calculated. The dye can be efficiently used in photography and in optical memory systems.