The association behavior of a maleic anhydride-grafted ethylene-propylene copolymer, EP(60/40), has been studied by several methods in hexane and tetrahydrofuran (THF). The concentration-dependent apparent hydrodynamic diameters (DBAR(H)) of EP(60/40) have been determined at 25.0-degrees-C by dynamic light scattering. DBAR(H) have been found to increase exponentially in both solvents. However, in THF DBAR(H) are much smaller and their increase with increasing concentrations of EP(60/40) is less steep than that in hexane. A pyrene-labeled (MBAR(w) = 40 000) EP(60/40), P-EP(60/40), has been prepared. Absorption, excitation, and emission spectroscopic measurements have provided evidence for the presence of ground-state pyrene complexes of P-EP(60/40) in hexane. In contrast, no evidence has been found for ground-state complex formation in THF by using these criteria. Treatment of the concentration-dependent excimer-to-monomer ratios has permitted the assessment of the equilibrium constants, aggregation numbers, and Gibbs free energy for the association of P-EP(60/40).