The present study systemically decolorized soy sauce using a membrane process to analyze the separation mecha-nism. An ultrafiltration (UF) membrane (NTU-2120) exhibited only slight decolorization ability. A nanofiltration (NF)membrane with a lower molecular weight cut-off and produced by sulfonated polysulfone (NTR-7400 series) ratherthan polyvinyl alcohol/polyamide (NTR-7250) had higher decolorization ability. The NF membranes rejected totalnitrogen by 17–24%, unsalted soluble solid content by 24–32%, reducing sugar by 25–43%, and amino acids by 10–25%.The NTR-7400 series membrane rejected lactic acid by 6–9%, and pyroglutamic acid by 11–21%; other quality indexeswere maintained. In the NF membrane processes, higher rejection of acidic amino acids than neutral and base aminoacids was observed. The separation performance was governed by the electrical effect as well as the sieve effect. Soysauce color could be controlled by blending NF membrane-processed soy sauce with feed soy sauce. Color can bematched to preference in accordance with dishes by suitably blending NF membrane-processed soy sauce with feedsoy sauce.