This article explores photophysical properties and aggregation behaviors of conjugated polymer, poly[2-methoxy, 5-(2'-ethylhexyloxy)-1,4-phenylenevinylene](MEH-PPV), in various solvent-nonsolvent systems by utilizing UV/vis absorption and photoluminescence (PL) spectroscopy. The isolated chains of MEH-PPV dispersed in solvents including dichloromethane, chloroform, and tetrahydrofuran adopt either extended or collapsed conformations depending on local polymer-solvent interactions. Aggregation of the MEH-PPV in these solvents is induced by addition of a poor solvent, cyclohexane. The formation of aggregates is indicated by the appearance of distinct red-shift peaks in the absorption and PL spectra. The degree of aggregation in each solvent-nonsolvent system is compared by means of absorbance and PL intensity of the aggregate bands. In early stage of the aggregation, the amount of aggregates in system is controlled by the solubility of polymer. When the polymer chains are forced to densely pack within assembled particles by increasing ratio of cyclohexane to 99 v/v %, the conformation of individual chain plays important role. We have found that the extended chains facilitate the aggregation in the assembled particles. Increasing chain length of polymer promotes the aggregation in early stage and densely packed particles. Size distribution of the assembled particles is also found to depend on the choice of solvent. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 894-904, 2010