Self-assembly of KHCO3 fibers is observed when glassy oligomerized films of poly[(aminopropyl)siloxane] containing K+ ions, denoted K+/poly-APS, are exposed to CO2 and H2O. The fibers are crystalline, narrow (0.4-3 mum diam), high aspect ratio (up to at least 300), and, on the basis of Raman spectroscopy, dominated by KHCO3. The fibers contain fluid that is dominated by aqueous potassium formate (KOOCH). A multistep phenomenological model is proposed to account for the self-assembly.