AbstractIntramolecular excimer formation between pyrenes attached to the ends of a poly(ethyleneoxide) sample of nominal molecular weight 9600 has been examined in twelve solvents. In ten non‐protic solvents the excimer to pyrene locally excited fluorescene intensity ratioIE/IMis inversely proportional to solvent viscosity, as expected for a diffusion‐controlled process. Similar results were obtained for 〈k1〉 values obtained from flash‐photolysis studies, where 〈k1〉 is the average rate constant for end‐to‐end cyclization in the polymer. In water and methanol, the extent of excimer emission and the rate of intramolecular excimer emission and the rate of intramolecular excimer formation were substantially greater than one would infer on the basis of solven