Pyrene-labeled poly(ethylenimine) (PEI) containing different amounts of pyrene per polymer were prepared by reductive amination of PEI with l-pyrenecarboxaldehyde. Aqueous solutions of the pyrene-labeled PEI (PEI-Py) exhibit typical pyrene monomer and excimer emission, and the excimer to-monomer ratio I-E/I-M decreases with decreasing gr content of the macromolecule. The excimer emission arises from preassociated pyrene aggregates. Both the monomer and excimer emission intensities increase with decreasing pH of the solution. A decrease in pH : of the solution from pH = 10 to pH approximate to 1.5 led to approximately a 100-fold increase in I-M, whereas I-E increased just 2- to 3-fold. A major feature of this system is the quenching of the Py fluorescence by ami:no groups of the PEI. This quenching depends on the degree of protonation :of these groups. Model experiments with pyrenemethanol show that triethylamine is 20 times more effective as a quencher than diethylamine in water, implying that most of the quenching in PEI is due to the tertiary amine groups. More interesting is the relative insensitivity of the excimer emission to this quenching process. This selective quenching may be due to the lower energy of the excimer state or the environment of the preassociated pyrenes in the polymer.