Figure shows the time-dependent UV spectrum recorded in a UV-DOSY measurement. The bands around 205 nm is due to caffeine, the bands around 325 nm and 285 nm to CGA. Due to the overlap of the species spectra, we cannot determine the individual diffusion coefficients by analyzing selected wavelengths as we did for the mixed dye solution above. To convert the experimentally recorded wavelength-and time-dependent absorption A(λ,t) to a DOSY spectrum, we perform a global least-squares fit of equation 3 to the two-dimensional data set, where the diffusion coefficients D i and the spectra A i (λ) of the two species are treated as fit parameters (note that the number of data points is much larger than the number of free parameters by a factor of approximately the number of time points, so the fit is well defined). From the least-squares fit (shown at selected wavelengths as the curves in Fig. ) we obtain the diffusion coefficients, D i , and the associated absorption spectra, A i (λ), of each species. In the bottom row we observe two signatures at 250 and 322 nm due to CGA, in the top row the peak at 277 nm due to caffeine. The observed diffusion coefficients for caffeine and CGA are 5.31±0.08×10 -6 and 4.06±0.05×10 -6 cm 2 /s, respectively, the difference being as expected from the different sizes of caffeine and CGA. For caffeine the diffusion coefficient has been reported previously, and our value corresponds to ∼70% of the literature value, the slower diffusion probably being due to the presence of added PEG in our solution. This example demonstrates that UV-DOSY can spectrally separate compounds with relatively small differences in diffusion coefficients (in this about 20%).