Fully hydrolyzed polyvinyl alcohol (FH-PVA) electrospun fibers with uniform diameters of less than 200 nm were fabricated by reducing the viscosity of FH-PVA aqueous solutions. A novel viscosity-modifier (hydrazine monochloride [HMC]) gradually reduced the viscosity of FH-PVA aqueous solution over a period of several days. This phenomenon is counter to the effect of the usual salt addition. After being stored for several days, the viscosity decreased by up to 60% compared with that of an equivalent pure FH-PVA solution. From small angle X-ray scattering (SAXS) and proton nuclear magnetic resonance (H-1 NMR) spectra observations it is evident that this effect results from the reconfiguration of hydrogen bonding. The viscosity control of FH-PVA solutions with HMC were used to electrospin highly uniform ultrafine fibers (diameter < 200 nm).