The proposed pseudomicellar character of five dissolved humic acids ha
s been investigated by monitoring the fluorescence behavior of a pyren
e probe added to the solutions. Fluorescence quenching by bromide in t
he presence of these humic acids was found to be consistent with a hyd
rophobic microenvironnent. The presence of metal ions, with either que
nching or nonquenching counterions, increased the pyrene fluorescence
of several of the humic acids. The results indicate that cations have
the property of increasing intramolecular micellization in these humic
acids, thereby minimizing quenching interactions. The proposed mechan
ism involves enhanced shielding of the probe rather than its expulsion
into the aqueous bulk as has been suggested previously. The humic aci
ds that show this micelle-like microorganization also display increase
d pyrene fluorescence upon heating, which can work in conjunction with
metal ion interactions. The observations suggest that a series of eve
nts take place that are comparable to the clouding behavior of nonioni
c detergent solutions. The microorganization progresses through the eq
uivalent of ''intramolecular clouding'' to the customary intermolecula
r analogue, leading to a separate humic acid phase.