For the first time, soy protein isolate (SPI)/ hydroxypropyl alkaline lignin (HPL) composites have been successfully prepared by mixing them in aqueous solution containing a small amount of glutaraldehyde as compatibilizer, and then compression-molded to obtain plastic sheets. The structures of the SPI/HPL composites were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and scanning electron microscopy, indicating the existence of amorphous networks and nanoscale HPL dispersion in the SPI matrix. When HPL content was lower than 6 wt %, the HPL-domain occurred in SPI/HPL composites with a dimension of about 50 nm, indicating a high interfacial activity. Differential scanning calorimetry analysis showed that the glass transition temperature of the SPI/HPL sheets increased from 62.5 to 70.4 degrees C with an increase of HPL content from 0 to 6 wt %. Moreover, the tensile strength of the SPI/HPL nanocomposite sheets with 6 wt % HPL and 3.3 wt % glutaraldehyde was enhanced from 8.4 to 23.1 MPa compared with that of the SPI sheets, suggesting that the nanoscale HPL dispersion significantly reinforced the SPI materials. (c) 2006 Wiley Periodicals, Inc.