We perform systematical viscometric investigations on the dilute solutions of a model electroluminescent conjugated polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV). Results on low-shear viscosity measurement reveal markedly close correspondence between viscometric and aggregation properties. Peculiar viscometric features of MEH-PPV solutions include (1) the polymer contribution to the solution viscosity exhibits a slow yet persistent decrease during constant-temperature aging, and (2) the same quantity exhibits thermal irreversibility as the solution has been subjected to a short-term thermal treatment. These viscometric features and corresponding aggregation properties are bridged using kinetic-theory/thermodynamic arguments and molecular dynamics simulations. Primary implications as to the effects of the choice of solvent, solvent quality, concentration, aging, and short-term thermal treatment on the aggregation properties agree fairly well with recent optical observations. Overall, the present study suggests that theological characterizations may be very useful for gaining additional insights into the aggregation properties for conjugated polymer solutions. (C) 2005 The Society of Rheology.