Three series of composite films based on polyimide and MWNTs were prepared by conversion of pyromellitic dianhydride and 4,4'-oxydianiline in the presence of the nanotubes, followed by thermal imidization. Carboxy- and amino-functionalized as well as unmodified nanotubes were used. It was demonstrated that just 0.5 wt.-% of nanotubes increased the tensile properties of the composite films distinctly. Surprisingly, a significant influence of the functional groups on the mechanical performance of the composite films could not be demonstrated. However, it was shown that functional groups may reduce the conductivity of the films. Furthermore, the influence of ultrasonication is discussed.