1. Fluorescence- and absorption spectra of a number of acridine dyes were measured at several concentrations, in different solvents and at various pH and temperatures. 2. In aqueous solutions metachromatic shifts are visible with all dyes — except acridine-yellow — with increasing concentrations, even when the dye is present as di- or trivalent ions in strongly acid solutions. 3. Under conditions where reabsorption of the fluorescent light is excluded: completely separated fluorescence and absorption spectra, or measurement of fluorescence in capillaries with 0.8 mm internal bore, metachromatic effects are absent. 4. Reasons are given to consider the hypothesis of specific aggregation (formation of dimeric dye-particles) as doubtful. In the case of the acridine dyes the optical properties of the monomeric dye ions are sufficient to explain the “metachromatic” shifts.