Ultrathin films of P3HT and P3HT/PCBM prepared on glass and ITO glass substrates by spin-coating were studied by means of highly sensitive absorption spectroscopy. For pristine P3HT films, we discuss the morphology and electronic structure at the interface between the substrate and the P3HT layer. For P3HT/PCBM films, we found that phase separation could be controlled by changing the concentration of the solutions used to prepare the films.