Regenerated Bombyx mori silk fibroin in formic acid was electrospun and the morphological, chemical and mechanical properties of these nanofibers were examined by field emission environmental scanning electron microscopy (FESEM), Raman spectroscopy (RS), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and tensile testing. FESEM indicated that the average fiber diameter was less than 100 nm and circular in cross section. This paper maps the silk fibroin molecular conformations of each step of the sample preparation and the electrospinning process. The secondary structural compositions (random and beta-sheet) of the fibroin were determined by FTIR and RS. The crystallinity index of the electrospun fiber, calculated as the intensity ratio of 1624 (beta-sheet) and 1663 (random) cm(-1) FTIR bands was higher than that of the pristine fiber. Raman spectra of the amide I (1665 cm(-1), random) to amide III (1228 cm(-1), beta-sheet) ratio of the electrospun fiber was less than that of the pristine fiber indicative of higher beta-shect content. The fiber crystallinity, determined by XRD, showed a lower value for the electrospun fiber. The electrospun fiber shows small but significant increases in the beta-sheet content in comparison with the pristine fiber. Dissolution of fibroin in formic acid enhances beta-sheet crystallization and may facilitate beta-sheet formation in electrospun fiber. The electrospun random silk mat had a Young's modulus, ultimate tensile strength and strain of 515 MPa, 7.25 MPa and 3.2%, respectively. (C) 2004 Published by Elsevier Ltd.