We report on the preparation and characterization of ultrafine polyamide-6 nanofibers by the electrospinning technique. The effect of electrospinning on the formation of ultrafine polyamide-6 nanofiber structure was examined. The morphological and structural characterizations and thermal properties of the ultrafine polyamide-6 nanofibers were investigated in comparison with bulk polyamide-6 pellets. In order to accurately characterize the ultrafine polyamide-6 nanofiber structure by direct identification of mass resolved components, we performed matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. Field emission scanning electron microscopy images revealed the presence of ultrafine polyamide-6 nanofibers bound between the main fibers. The diameter of the polyamide-6 nanofibers was observed to be in the range 75-110 nm, whereas the ultrafine structures consisted of regularly distributed very fine nanofibers with diameters of about 9-28 nm. The MALDI-TOF spectra showed the presence of protonated and sodiated ions that were assigned to polyamide-6 chains. © 2011 Society of Chemical Industry.