In the previous report5), it was suggested that “blocking effect” in dyeing of 6-nylon with acid dyes is attributed to the hydrophobic charactor of 6-nylon fibre.In the present report, the same problem is investigated from the dyeing mechanism of 6-nylon and acetylated 6-nylon with acid dyes.The dyes used are Orange 8, Orange II and Alizarine Saphirol A. 6-nylon used was acetylated with acetic anhydride under different conditions (Table 1). The adsorption isotherms of three dyes and a mixture dye for 6-nylon and acetylated 6-nylon were determined at 90°C, 72hr..The results obtained are as follow:1) From the results of the adsorption isotherm (Fig. 2-4), it seems reasonable to assume that the dyes are adsorbed by 6-nylon or acetylated 6-nylon by means of both ionic and non-ionic interaction; and the main part of adsorption by acetylated 6-nylon is attributed to the non-ionic interaction.2) Isotherm for a binary mixture of acid dyes on the acetylated 6-nylon is similar to that for disperse dyes on the normal 6-nylon having no “blocking effect” (Fig. 5-7).In view of the above facts, it seems most reasonable to conclude that “blocking effect” does not take place by a small amount of amino end group contained in the nylon, but by the balance of ionic adsorption and non-ionic adsorption to nylon with acid dye mixture.