In this work, a simple approach to prepare luminescent poly(3-hexylthiophene)-CdS nanocomposites to be employed in organic light emitting devices (OLED) devices is reported. The nucleation and growth of CdS nanoparticles were obtained by the thermolysis of a single Cd and S precursor dispersed in the polymer at three different temperatures of annealing: 240, 265, and 300 degrees C. In this way, it was possible to compare the properties of nanocomposites containing nanoparticles with different sizes. X-ray diffraction and transmission electron microscopy analyses confirmed the formation of CdS nanoparticles and gave information about the size, distribution, and morphology of the nanoparticles; monodispersive and very small nanoparticles with diameters below 2.5 nm were obtained at 240 degrees C. The application of such nanocomposites as emitting layers in OLED devices is discussed. Enhanced electrooptical properties were observed for the device containing the nanocomposite annealed at 240 degrees C with respect to the pure polymer based device. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 3624-3629, 2011