Mesoporous tungsten oxide nanofibers were synthesized via a 500 degrees C thermal treatment of composite nanofibers prepared by electrospinning an ethanol solution consisting of tungsten ethoxide, P123 triblock copolymer, and polyvinylpyrrolidone. The as-electrospun composites exhibited unwoven nanofibers with an average diameter of 233 nm and a smooth surface morphology. During the calcination process, the composite nanofibers were shrunk to 85 nm in diameter and converted into rough, wormhole-like nanofibers. These were formed by agglomerating polycrystalline WO(3) particles of 10-30 nm along the axial direction. Furthermore, a measured pore-size distribution indicated that this nanofiber mat had different types of mesosized porosities, which may have resulted from their wormhole-like structures and inter-fiber voids. In addition, it was observed to have the intra-grain porosity with the diameter of about 1.0 nm. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.