<p>Dissolved organic matter (DOM) in wastewater affects the operation and costs of water treatment processes but its elimination from complex systems is problematic because the mechanisms of degradation are not clear. This study applied three techniques to elucidate the degradation of DOM in two hospital effluent and landfill leachate under simulated sunlight and dark conditions. DOM composition and structure were examined through changes in optical parameters. Two-dimensional correlation spectroscopy of FTIR data demonstrated a difference in degradation sequence between the light and dark groups, particularly involving de-protonated carboxyl groups and overlapping O–H and N–H stretching of other functional groups. Under simulated sunlight, samples degraded in the order 3436 → 1643 → 1137 → 623&#xa0;cm⁻¹, but in the dark the sequence was 1404 → 1137 → 3436&#xa0;cm⁻¹, suggesting that the FTIR band associated with overlapping O–H and N–H stretching of compounds is particularly susceptible to light. Parallel factor modeling identified two humic-like (C1 and C3) and one protein-like (C2) components. C3 related to photodegradation of terrestrial DOM and microbial processing and was readily photodegraded. Two new fluorophores were generated during the photodegradation of the protein-like substances in the hospital wastewater. The untreated hospital wastewater had aromatic characteristics (shown by first-order UV–vis spectra) that were no longer evident after irradiation. The lowest absorption and fluorescence intensities were observed after 36&#xa0;days, suggesting one month may be the optimum duration for wastewater irradiation treatment. Probably, this slow photodegradation of DOM would be practical application when the wastewater is exposed to large areas of natural sunlight.</p>