A maleated ethylene-propylene random copolymer (EP) was reacted with 1-pyrenemethylamine to yield a pyrene labeled EP, and the photophysical properties of the pyrene label were used to determine the level of aggregation between the resulting succinimide groups as a function of the composition of hexane/toluene mixtures. In hexane, the polar succinimides bearing the pyrenyl pendants form aggregates whose presence can be inferred from fluorescence and UV-vis absorption measurements. Addition of toluene is found to melt the pyrene aggregates. The aggregation induced by the polar succinimide groups was shown to occur intramolecularly at polymer concentrations of 0.1 g/L by fluorescence resonance energy transfer. The level of pyrene aggregation (f(agg)) was determined quantitatively and could be followed as a function of the volume fraction of hexane in a series of toluene/hexane mixtures. Upon addition of toluene, the parameter f(agg) decreased rapidly to a finite value different from 0.0 due to the inherent clustering of succinic anhydride groups occurring during the maleation of EP. This lowest f(agg) value was reached for a mixture containing 60 vol % of hexane. The ability of toluene at melting the pyrene aggregates was shown to have a profound effect on the rheological properties of the polymer solutions.