In summary, it was demonstrated that mean free path corrected Mie theory affords a quantitative description of the absorbance of GNPs in the size range from 5 to 100 nm where a good agreement with the experimental data was found. It was shown that both the size and the concentration of GNPs can be determined directly from UV-vis spectra using eqs 10-13 for the calculation of d and eq 14 for the calculation of N together with the corresponding fit parameters stated in the text. Tabular material presented in the Supporting Information allows the readerทto determine size and concentration of GNPs without the need to perform calculations.