Water-insoluble organic UV filters such as 2,2'-methylene-bis(6-(2H-benzotriazole-2-yl)-4-(1,1,3,3-tetramethylbutyl)-phenol) (MBBT) can be prepared as aqueous dispersions of nanoparticles. The particles consist of the respective UV absorber molecules and show strong UV absorbance. Because there is a certain solubility of such UV absorbers in organic solvents, it is possible to measure the absorbance spectrum in solution also, for instance, in ethanol or dioxane. The UV spectrum of the aqueous dispersion shows a significant bathochromic shift of the long-wavelength band with an additional shoulder. For the understanding of the observed changes of UV-vis spectra of this UV absorber, either dissolved in an organic solvent or dispersed as nanoparticles in water, density functional theory (DFT) calculations were carried out with the respective monomer and aggregates of MBBT molecules in different media. The calculated UV-vis spectra of isolated, that means dissolved, MBBT molecules in ethanol and in dioxane agree well with the experimentally observed ones. The observed changes in the shape and position of experimental UV-vis spectra in aqueous dispersion cannot be explained with the solvent effect alone. It was found that the studied molecules could form stable energetically favorable pi-stacked dimers, which show UV-vis spectra in reasonable agreement with those experimentally observed in aqueous dispersion. Such aggregates of MBBT are most likely the reason for the observed bathochromic shift in the UV-vis absorption spectrum. In addition, the mechanism of the photochemical deactivation of the excited MBBT molecules was studied in detail with time-dependent DFT in dioxane and in water. The energetically most favorable pathway for the deactivation of absorbed energy by MBBT occurs through intramolecular enol-keto tautomerization in the first excited singlet state.