In this work, we use UV-Vis and Raman spectroscopy to correlate the intensity of selected transitions to the onset of aggregation phenomena. Through TDDFT calculations, we rationalize the formation of H-aggregates and their influence on the observed changes in the UV-Vis spectra. A correlation between Raman intensity and the molar absorption coefficient is experimentally observed and theoretically rationalized. We develop this method by considering Disperse Orange 3 (DO3), a well-known push–pull azobenzene dye with strong optical absorption in the blue–green region of the visible spectrum, and the known tendency to form H-aggregates.