Dissociation constants and heat of dissociation of dimeric associates of an anionic oxacarbocyanine dye, potassium 3, 3'-bis [3-sulfopropyl]-5, 5'-diphenyl-9-ethyloxacarbocyanine (DYE), in aqueous ethanol solutions have been determined from the monomer band absorbance of the dye measured at different temperatures and dye concentrations at which the monomer is the major solute species and disturbance of monomer absorbance measurements by associated species is fairly small. In 10% ethanol, the dissociation constant was 2×10^-5 mol/dm³ (20°C)-7×10^-5 mol/dm³ (40°C), and the heat of dissociation was about 10 kcal/mol (42kJ/mol), suggesting a higher dimer stability of DYE than that of typical cationic thiacarbocyanines.