Fluorescence spectra of polyacenaphthylene (PAN) were determined in dilute solution at a constant temperature, 25.0-degrees-C. Solvents were prepared by mixing two good solvents in different proportions. Solvent viscosity (eta) and polymer intrinsic viscosity ([eta]) were also measured at 25.0-degrees-C for each solvent composition. Solvent mixtures were chosen to show a monotonous change of only solvent fluidity (eta-1) (benzene-dioxane) and both eta-1 and [eta]-1 simultaneously (tetrahydrofuran-dioxane, chloroform-tetrahydrofuran, and cyclohexane-diethyl ether). A single solvent, toluene, was also used. The dependence of the excimer to monomer fluorescence ratio on solvent viscosity and segmental density (directly proportional to [eta]-1) has been discussed in terms of a kinetic scheme which takes into account energy migration, ground state preformed dimers, and unquenched monomeric units. It predicts that the excimer to monomer fluorescence ratio depends on the product eta-1[eta]-1.