Nanofibers were produced by electrospinning aqueous alkaline solutions containing different mass ratios of soy protein and lignin in the presence of poly(ethylene glycol) coadjutant, all of which presented shear thinning behavior. SEM revealed that the addition of polyethylene oxide as a coadjutant indeed facilitated the formation of defect-free fibers whose diameter increased with lignin concentration, in the range between approximate to 124 and approximate to 1400 nm. Favorable interactions between lignin and soy protein were identified from data provided by differential scanning calorimetry. In addition, an increased hydrogen bonding and the loss of secondary structure of the proteins as the lignin concentration increased were observed from the disappearance of amide II (similar to 1500 cm(-1)) and III (similar to 1400-1200 cm(-1)) bands and a red shift of amide I band in the FT-IR spectrum. The unfolding of the protein contributed to a better interaction with lignin macromolecules, which further improved the electrospinning process. It is concluded that mixtures of lignin and soy proteins, two major renewable resources with interesting chemical features, are suitable for the development of composite sub-micron fibers. (C) 2014 Elsevier B.V. All rights reserved.