This work provides data on the interaction of fulvic acids extracted from a Spodosol soil, dissolved in water, with other aqueous solutions of various pesticides (amitrole, chlordimeform, diquat, and paraquat) in diverse concentrations. A strong interaction was observed, precipitates appearing that were more abundant with greater concentrations of the pesticide used. Both the weights of the precipitates obtained and the coloring of the equilibrium solutions indicate that some FA persist in solution in spite of an excess of pesticide. The quantity of precipitate followed the order: diquat > paraquat > chlordimeform >> amitrole. The interaction mechanisms between the FA and the pesticides were determined by infrared spectroscopy. The adsorption of these pesticides by FA is a cation-exchange process to which are added H-bonding and charge-transfer mechanisms.