Polyaniline-polypyrrole (PANI-PPy) nanofibers with high aspect ratios have been synthesized by a one-step, surfactant-assisted chemical oxidative polymerization from mixtures of aniline (An) and pyrrole (Py) monomers. PANI-PPy nanofibers synthesized with an excess of either PANI or PPy show similar spectral (UV-vis and FT-IR) characteristics as the individual homopolymers, whereas nanofibers from an equimolar mixture of An and Py display unique spectral characteristics. PANI-PPy nanofibers undergo a spontaneous redox reaction with metal ions to produce metal nanoparticles with various morphologies and/or sizes. These findings may open new opportunities for synthesizing functional polymer nanofibers and metal nanoparticles with controllable sizes and/or morphologies.