Sulfur dyes and method of preparing same: membrane process

A process for the production of an aqueous dispersion of an oxidized sulfur dye having a lowered content of inorganic salt which comprises subjecting an aqueous suspension of oxidized sulfur dye particles which also contains inorganic salt to separation by a semipermeable membrane having a pore size giving a cut-off so lower than molecular weight 500 which prevents passage therethrough of the desired size dye particles but permits passage therethrough of water and inorganic salts.