The pK_a values for the 5- and 6-aminoindoles were determined from potentiometric titration curves and from ¹³C NMR data on the total change of the chemical shifts of the carbon atom signals on protonation of the amino group. The pK_a values obtained (5.99 and 5.53) were higher than those of aniline (3.92) or -naphthylamine (3.39).For Communication 130, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, 766–769, June, 1988.