Due to their unique properties, carbon nanotubes (CNTs) have been used as thin electrodes in plastic optoelectronic devices. In many applications, it is required that CNT electrodes be transparent, conductive and flexible, and most importantly, mechanically stable with good adhesion to the polymeric substrate. In this paper, we report on achieving SWCNT transparent and conductive films with excellent adhesion to polyethylene terephthalate, without any binder, by a simple and rapid post-treatment process. It was found that the best adhesion was achieved upon treating the films with acetic acid and formic acid, and with solutions containing 1-70% HNO3. Morphological evaluations indicate the unique adhesion due to the SWCNT becoming partly embedded within the polymeric substrate during the post-treatment process, thus yielding flexible conductive films with high transparency.