Steady-state fluorescence has been used to study the dependence of the intramolecular formation of excimers in media of differing viscosity, eta, for model compounds of polyesters containing naphthalene groups. The model compounds are derived from 2-naphthalene carboxylic acid as rigid units and glycols, HO - (CH2)(m)-OH, m = 2-6, as flexible spacers. The ratio of the intensity of excimer and monomer emissions, I-D/I-M, exhibits two regimes : a regime at high eta where I-D/I-M shows an odd-even effect with m, with maxima at odd m, and a regime at low eta where the odd-even effect is lost, and the maximum values are obtained for m = 3 and 6. Calculations performed for the equilibrium state using the rotational isomeric state model and Molecular Dynamics (MD) simulations allow the rationalization of the behavior of I-D/I-M with m in the media of high and low eta, respectively.