This study explores roles of initial solvent on the formation of conjugated polymer nanoparticles (CPNs) and their photophysical properties. Stable aqueous CPN dispersion of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylvinylene)(MEH-PPV) and regioregular poly(3-octylthiophene)(rr-P3OT) are prepared via reprecipitation technique. This preparation method involves the injection of polymer solution in organic solvents into an excess amount of water. We demonstrate that water solubility of the initial solvent is a major factor dictating mechanism of the CPN formation. Dichloromethane (DCM) and tetrahydrofuran (THF), possessing very different water solubilities, are used as initial solvents in this work. The resultant CPNs exhibit quite different sizes and photophysical properties. The preparation of MEH-PPV nanoparticles from DCM solution provides average size of about 127 nm. Their absorption and photoluminescence (PL) spectra shift to higher energy region compared to those of the isolated chain. When the THF solution is used, opposite results are observed. Average size of the nanoparticles decreases to about 40 nm. Significant redshift of their absorption and PL spectra is also detected. Detailed data analysis indicates that the individual chain conformation and degree of segmental aggregation within the CPNs are quite different. This leads to drastic discrepancies of their photophysical properties. The use of DCM and THF as initial solvents provides the MEH-PPV nanoparticles with green (lambda(max) = 535 nm) and red (lambda(max) = 590 nm) photoemission, respectively. The investigation of rr-P3OT provides consistent results. Our study offers a new and simple route to control size and photophysical properties of CPNs by careful selection of the initial solvents. (C) 2013 Elsevier Inc. All rights reserved.