A process for preparing an asymmetrical indocyanine dye comprising the steps of: a) reacting a first quaternised indolenine or substituted indolenine with a compound of the formula (II) 1 or hydrochloride thereof, wherein n is 0 or 1 Ph is phenyl or substituted phenyl X is hydrogen, halogen or alkyl, preferably chlorine, in a solvent selected from the group consisting of acetic acid, acetic anhydride and mixtures thereof in the presence of acetyl chloride, to obtain an intermediate hemicyanine, and b) further reacting said intermediate hemicyanine with a second quaternised indolenine or substituted indolenine different from said first indolenine.