However, comparatively little information about the size of molecules or molecular assemblies can be obtained from the UV/Vis spectrum, because electronic transitions generally occur in subunits of the molecules, such as the aromatic residues in proteins and the purine and pyrimidine bases in DNA and RNA. To obtain both size and UV/Vis-spectral information, chromatographic methods are very well suited, and the combination of liquid chromatography and UV/Vis detection can be used for e.g. biopolymer size characterization. However, these methods have limitations associated with the molecular size as well as the need for known calibrants to determine the relation between the retention behavior of the sample molecules and their size. Here, we present a different method to simultaneously obtain the size and UV/Vis spectrum of molecules or particles in solution. Inspired by the elegant NMR concept of Diffusion-Ordered Spectroscopy (DOSY), we demonstrate how a simple extension of a standard UV/Vis spectrometer can be used to simultaneously characterize the electronic structure and the size of a molecule or particle. Similar to NMR-DOSY, the resulting two-dimensional spectra have absorption wavelength on one axis, and diffusion coefficient on the other, and if there are several species with different sizes present in a solution, their individual spectra are separated and sorted by size. This method is an extension to visible and UV wavelengths of our previous work on infrared and Raman diffusion-ordered spectroscopy which was limited to infrared-transparent liquids. The method relies on the fact that the diffusion coefficient D of a molecule (or particle) is inversely proportional to its hydrodynamic radius R through the Stokes-Einstein relation. In Figure we show how this effect can be used to simultaneously determine the sizes and UV/Visspectra of the species present in a solution. We simultaneously inject the sample solution and the pure solvent into a thin space between two UV-transparent windows, in such a way that the two liquids are in contact in a parallel flow. At t = 0, we stop the flow. The solute molecules (red and blue dots in Fig. ) then diffuse from the solution-filled half into the solvent-filled half, each at a different rate determined by its diffusion coefficient. We measure the time-dependent UV/Vis absorption spectrum at a position in the sample where there was initially only solvent.