Manganese oxide nanoparticles-polyethylene oxide nanofibers as organic-inorganic hybrid were prepared via in situ electrospinning. Thus, electrospinning of polyethylene oxide solution with different manganese chloride concentration was carried out in gaseous ammonia atmosphere containing oxygen. The reaction of manganese chloride with ammonia produces manganese hydroxide, and then oxygen in the reaction media reacts with manganese hydroxide to yield manganese oxide. These two reactions occur during fiber formation. Transmission electron microscopy and scanning electron microscopy showed that manganese oxide (MnO2) nanoparticles were formed on the produced nanofibers of 100-600 nm in diameter. The existence of the formed MnO2 was also proved by X-ray diffraction analysis, and it showed that the lambda-MnO2 nanoparticles were produced. Differential scanning calorimetry (DSC) analysis was used to determine the melting point and to calculate the crystallinity of the produced hybrid nanofibers. The DSC and thermogravimetric analysis results of the obtained nanofibers were compared with those of the nanofibers produced in electrospinning of pure polyethylene oxide solution. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 243-249, 2010