In this study, we show that several UV–Vis absorbance, steady-state and time-resolved fluorescence parameters of a series of dissolved humic substances (DHS) from different sources (e.g. terrestrial fulvic and humic acids, and humic acid-like molecules extracted from composted and vermicomposted wastes) correlate with the molar absorptivity at 280 nm per mole of organic carbon (ε280), which in turn is proportional to the molecular complexity (e.g. molecular size, aromaticity and oxidation degree) of the DHS. Both absorbance and fluorescence spectral responses were sensitive to the molecular complexity associated with the maturation degree of the DHS. Depending on the DHS, different emitting responses by excitation at the UVA (340 nm) and VIS (460 nm) regions of the absorption spectra were observed. The results were explained in terms of the extent of intramolecular electronic interactions between electron donor groups, such as polyhydroxylated aromatics and indoles, and more oxidized acceptor groups (e.g. quinones or other oxidized aromatics) as the molecular complexity of the DHS increased.