The present research unveiled that acidity dominates while transition metal ions harmonize with the light absorption properties of humic-like substances (HULIS). Cu²⁺ has quenching effects on HULIS by complexation, hydrogen substitution, or electrostatic adsorption, with aromatic structures of HULIS. Such effects are less pronounced if from Mn²⁺, Ni²⁺, Zn²⁺, and Cu²⁺. Oxidized HULIS might contain electron-donating groups, whereas N-containing compounds might contain electron-withdrawing groups.