A potential mechanism for the interactions of dissolved humic acids with nonpolar micropollutants was assessed by monitoring the fluorescence of pyrene added to aqueous humic acid solutions. The results support the concept of the cation-assisted formation of intramolecular domains in humic acids and a partitioning mechanism for probe sequestration in these domains. Fluorescence enhancement of pyrene in the presence of different humic acids showed significant variations, leading to the conclusion that large soil humic acids are more effective in isolating small molecules than smaller aquatic or structurally rigid humic acids. Solution variables such as ionic strength, pH, and type of cation present also exerted an influence. Stern-Volmer plots showed that salts cause humic acid to become less quenching towards pyrene fluorescence, which is ascribed to a change in the quenching mechanism. Precipitation studies indicated that the charge of the cation is the most important parameter in the salt-promoted pseudomicellization of humic acids.