A dye-producing plant using nanofiltration membrane technology for the desalting and concentrating of aqueous dye was designed and built in China in 1993. The design features, operating experience and typical results such as membrane selection, element style, flow diagram, operating pressure, salt passage through membrane and change of permeate flux are described in detail. The membrane system can produce aqueous dye with more than 25% dye and about less than 1% salt content. The improved process using nanofiltration membrane technology is continuous and automatic in operation. It is not very labor intensive, and produces dye products with high purity and consistent quality.