Quenching of the excited singlet states of a water-soluble,sterically congested tetraarylpyrene, 1,3,6,8-tetralds(2,6-dimethy1-4-(alpha-carboxy)methoxyphenyOpyrene (Py4C), by a series of iodide salts has been investigated by steady-state and time-resolved fluorescence measurements. Access to the pyrenyl group of Py4C is restricted sterically as a result of the four flanking (2,6-dimethylphenoxy)acetic acid groups and the energy costs associated with their rotation. Deprotonation of the carboxylic acid groups of Py4C permits examination of ion ion electrostatic interactions on the rates of quenching by iodide salts in which different steric and electrostatic factors are introduced by varying the cationic portions. At the same concentrations and with the same cations, chloride anions are ineffective quenchers. The quenching rate constants of Py4C by iodide are found to correlate linearly with the ionic radii of the cations and their enthalpies of hydration. These correlations are discussed in terms of the Hofineister series. Furthermore, the results indicate that the cations that flank Py4C decrease the quenching efficiency of iodide through polarization and shielding effects (i.e., lowering the effective charge), which isolate to varying degrees the pi-system. The effects of the different cations on quenching the fluorescence of a simpler and sterically unencumbered pyrenyl derivative, 1-pyrenylbutyric acid (PyBu), by iodide are much smaller. Overall, the results with Py4C indicate that the fluorescence quenching efficiency by iodide is influenced by direct interactions with the cations associated with the carboxylate groups of Py4C and not the solvation of water molecules. This observation is germane to a topic of current debate: Are the effects of the cations more closely related to bulk water properties or to direct ion ion interactions? The conclusions obtained from these studies are applicable clearly to a wide variety of other systems in which ion pairing influences cooperative or inhibitory interactions.