Nanocomposites containing controlled (in the course of synthesis) concentrations of lead sulfide nanoparticles dispersed in the polymer matrix were prepared by the method of gas-phase polymerization on the surface of an active monomer, paracyclophane, which was codeposited with lead sulfide molecules. According to wide-angle X-ray diffraction measurements, the prepared nanocomposites contain lead sulfide clusters of 4 nm size, which are localized in the amorphous regions of poly-p-xylylene. Size distribution curves for lead sulfide particles as calculated on the basis of the Fourier analysis of crystallographic X-ray reflections and the volume distributions functions of particles as determined from small-angle X-ray scattering data are reported. A mechanism for the formation of lead sulfide nanoparticles in the poly-p-xylylene matrix is proposed on the basis of analysis of the X-ray scattering curves.