The morphological evolution of thin composite films based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) upon annealing has been studied by means of transmission electron microscopy (TEM), scanning near-field optical microscopy (SNOM), and confocal Raman microscopy. This system currently is the most promising candidate for high-performance polymer solar cells. TEM bright field and SNOM topography measurements show that segregation and large-scale crystallization of PCBM take place upon thermal annealing. Additional SNOM optical absorbance measurements using laser irradiation sources with two different wavelengths in the visible light range and Raman microscopy are able to detect the component distribution within the thin composite film, which in detail demonstrates high diffusive mobility of PCBM upon annealing.