The aggregation properties of 4-(1-pyrene)butanoate in aqueous media have been quantitatively investigated. For the pyrene chromophore ground state, the monomer-dimer equilibrium was monitored using NMR methods? yielding a dimerization constant of 150 M-1 for pyrenebutanoate in alkaline water. m addition, the excited-state aggregation was studied by time-resolved emission spectroscopy. The excimer formation constant in water, which represents the equilibrium established by interaction of excited- and ground-state species, was determined to be 1.6 x 10(4) M-1 (about 100-fold larger than the ground-state dimerization constant). The role of solvent polarity and the influence of an aromatic cosolvent were examined in order to determine the types of interactions that contribute to the driving force for aggregation. The ground-state aggregation propensity of pyrenebutanoate was significantly diminished upon addition of methanol or pyridine to aqueous solutions, indicating that hydrophobic and/or pi -stacking interactions play a role in the aggregation processes. Similar trends were observed also in the case of excited-state aggregation when the organic cosolvents were included. The investigation provides a quantitative assessment of the thermodynamics of interaction of pyrene fluorescence probes that are widely used in aqueous media in biophysical studies.