Nylon 6.6 was treated with anionic synthetic tanning agent at different pH values, temperatures and liquor ratios to determine exhaustion properties. Treatments using syntan were conducted to the undyed nylon 6.6 fiber and three acid dyeings and then samples of the bath solution were subjected to HPLC analysis. Mass analysis was considered to determine the syntan components. In addition, the effect of applications using syntan and syntan/cation in improving the stain resistance was observed. The optimum treatment condition of syntan was achieved and the major component of syntan, being exhausted to the nylon substrates, was determined. In terms of stain blocking effect, a significant improvement was achieved by the syntan and syntan/cation treatments.