From the concentration dependence of the spectrum of 9-aminoacridine hydrochloride in neutral aqueous solution we determined the spectrum of the dimeric dye species. Two further 'dimeric' spectra' of the 9- aminoacridine chromophore have been obtained as the low- and high-temperature forms of N,N′- di (acridin-9-yl)hexane-1,6-diamine in aqueous solution. We interpreted the dimer spectra in the region between about 245 and 275 nm in terms of vibronic exciton theory and discussed the implications of the results to the structure of the dimers . Application of vibronic exciton theory to the visible region of the dimer spectra has resulted in at most qualitatively acceptable fits only, due to the breakdown in this region of the spectra of the single dominant vibrational mode approximation.