Morphology, spectroscopic properties and molecular structure defects are characterized and compared for two types of poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene]vinylene} (MEH-PPV) -Au nanoparticles (NPs) nanocomposites prepared by different procedures known for nanocomposites with other polymers: (a) nanocomposites M, prepared by mixing a dichloromethane (a) CM solution of MEH-PPV with Au nanoparticle organosol in DCM, and (b) nanocomposites R, prepared by the in situ interfacial reduction of Au3+ salt with NaBH4 in the presence of polymer. In nanocomposite M the molecular weight (MW) distribution as well as molecular structure of parent MEH-PPV remains practically intact and the polymer forms a matrix in which, at higher polymer concentrations, Au NPs are almost uniformly distributed and preserve the particle-size distribution found for the parent sol. The aggregation of Au NPs is observed as the polymer concentration in the nanocomposite M is decreased. For nanocomposites R: (i) a broad particle-size distribution and formation of Au aggregates, (ii) a slight decrease in molecular weight of MEH-PPV with respect to the parent polymer, and (iii) a decrease in the extent of conjugation of MEH-PPV chains fairly exceeding that observed for MEH-PPV chains degraded to the same MW values in air at ambient light were observed. These features of nanocomposites R are shown to be a consequence of side reactions of MEH-PPV with H[AuCl4]/TOAB complex in the reaction mixture prior to the addition of the reducing agent. Unlike the case of nanocomposites M, the higher the polymer concentration, the larger is the mean size of Au NPs that tend to form larger aggregates in nanocomposites R. (C) 2008 Elsevier B.V. All rights reserved.