An association index based on the fluorescence anisotropy of a diphenyloxazole probe is introduced as a quantitative parameter for the estimation of the associations of humic acid with nonpolar organic compounds. The variations of the index were consistent with the behavior of dissolved humic acids in response to solution conditions. These included salt concentration, pH, humic acid concentration, probe concentration, and type of humic acid. The evidence provided further support for the formation of humic pseudomicelles that have the ability to sequester hydrophobic species in the solution environment. The molecular parameters governing this behavior include size and flexibility. The association indices of humic acid size fractions suggest the existence of an intermolecular component in the association process, leading to pseudomicellar behavior that depends on the polydispersity of the humic acid solution.