Mixtures of poly(3-octylthiophene) (P3OT) with graphite nanoparticles have been investigated by scanning force microscopy (SFM) techniques. The morphology as well as the mechanical and electrical properties of the blends has been characterized at the nanoscale level as a function of the carbon nanoparticle content in the blend. An increase in the concentration of carbon nanoparticles results in an increase in the surface roughness of the blend and the appearance of distinct regions with well-defined electrical and mechanical properties. At intermediate concentrations (5-10 wt % of carbon nanoparticles), the samples show pure P3OT regions, as well as round regions containing a mixture of the polymer and carbon nanoparticles, while at higher concentrations (> 15 wt %), the entire sample is composed of this mixture. The interface between the two regions has been studied by electrostatic scanning force microscopy (ESFM) as a function of the applied tip-sample voltage. ESFM provides evidence for the creation of new electronic states at the heterojunction. The observed results can be qualitatively explained in terms of the electronic properties of the individual molecular components, P3OT, functionalized graphite nanoparticles, and their corresponding heterojunction. The implications of these results for organic polymer solar cells are also discussed.