Fluorescence probe studies show that sodium taurocholate (NaTC), a trihydroxy bile salt, can be used not only to increase the water capacity of AOT reversed micelles but also to modify the micellar organization in order to alter molecular mobility, permeability, and microenvironmental polarity at the interfacial regions. Measurements of fluorescence intensity, spectra, anisotropy, and lifetimes were used in combination with absorption spectroscopy and classical and dynamic light scattering to provide insight into the effects of NaTC on microenvironmental heterogeneity experienced by the probes, which included the hydrophilic fluorescein and lipophilic ANS and NBD-hexanoic acid (NBD-HA), in the novel, four-component system of heptane/(AOT, NaTC)/water. Small, amounts of NaTC were found to disrupt the AOT micellar organization, particularly at the AOT/water interface. At higher NaTC, there is an increase in micellar size as well as structural organization that leads to higher probe anisotropy. At the AOT/water interface, NaTC creates a more polar, water-like environment for fluorescein and decreases the efficiency with which fluorescein is quenched by Tb3+, while at the heptane/AOT interface, NaTC creates more apolar environments for ANS and NBD-HA and increases the efficiency with which ANS is quenched by CCl4. At no or low NaTC, heating to 45 degrees C has little effect on the micelles; at higher NaTC, heating expands the micelles and disrupts their organization.