The nanostructure of PbSe-ZnSe composite thin films prepared by the hot-wall deposition (HWD) method was investigated using small-angle x-ray (SAXS) scattering. The SAXS profiles indicate the formation of two kinds of nanoparticles: large nanoparticles that vanish and small particles that increase in size with increasing temperature. At high substrate temperatures, the volume fraction of all the nanoparticles estimated from SAXS is consistent with that of PbSe obtained by chemical analysis. This shows that PbSe forms nanoparticles at high substrate temperatures. On the other hand, the same analysis for the volume fraction at low substrate temperatures reveals that the chemical composition of the nanoparticles differs from PbSe. Pb nanoparticles are probably formed at low substrate temperatures and disappear with increasing substrate temperature.