We have successfully prepared a series of blend membranes from cellulose and soy protein isolate (SPI) in NaOH/thiourea aqueous solution by coagulating with 5 wt % H2SO4 aqueous solution. The structure and properties of the membranes were characterized by Fourier transform infrared spectroscopy, ultraviolet-visible spectrometry, dynamic mechanical thermal analysis, scanning electron microscopy (SEM), transmission electron microscopy, and tensile testing. The effects of SPI content (W-SPI) on the structure and properties of the blend membranes were investigated. The results revealed that SPI and cellulose are miscible in a good or a certain extent when the SPI content is less than 40 wt %. The pore structure and properties of the blend membranes were significantly improved by incorporation of SPI into cellulose. With an increase in W-SPI from 10 to 50 wt %, the apparent size of the pore (2r(e)) measured by SEM for the blend membranes increased from 115 nm to 2.43 mum, and the pore size (2r(f)) measured by the flow rate method increased from 43 to 59 run. The tensile strength (sigma(b)) and thermal stability of the blend membranes with lower than 40 wt % of W-SPI are higher than that of the pure cellulose membrane, owing to the strong interaction between SPI and cellulose. The values of tensile strength and elongation at break for the blend membranes with 10 wt % of W-SPI reached 136 MPa and 12%, respectively. The blend membranes containing protein can be used in water because of keeping sigma of 10 to 37 MPa. (C) 2004 Wiley Periodicals, Inc.