Potentials zeta were measured in (i) distd.-H2O and buffered (pH 4.5) 0.1-0.8 g/L acid dyestuff solutions, (ii) polyamide PA6 fibers suspended in pure H2O and in aq. 0.1-0.8 g/L acid dyestuff solutions, (iii) dyed polyamide fibers, 150 mg/100 mL distd. H2O. Polish mono (Red G, Acid Turquoise Blue A), di (Acid Ponceau 2RL, Acid Black A, Acid Violet R), and trisulfonic (Acid Ponceau 4R) dyestuffs and nonsulfonic Polane Yellow ER were studied. The PA6 yam, 2 g, was dyed with 0.1-0.8 g/L dyestuff in a 3% AcONH4 + 2% AcOH (fiber wt. basis) buffer. In the aq. 0.1-0.8 g/L solutions, the potential was strongly neg., -550 to -850 mV, proportionally to the number of sulfonic groups, and in the dyed fiber suspensions still more negative (4500 mV, Acid Black A). At lower pH, more dyestuff was adsorbed and zeta rose.