The acid dissociation constants, K h, of the hexaaquacobalt(III) and -manganese( III) ions are reported under conditions which minimize the problems of instability of these species at modest acidities ( pH 1 -4). A rapid and convenient preparation of aqueous Co(III) has been used, and the latter has been reacted with aqueous Mn(II) to form aqueous Mn( III). The noncoordinating buffer H2PIPPS (1,4-bis(3-sulfopropyl)piperazine) has been used to react with H+ and provide some control of the pH. Spectrophotometric changes, after stopped-flow mixing for Co( III) and conventional mixing for Mn( III), were used to determine the K h values. The p K h values are as follows: Co( III), 3.10 +/- 0.07 (6.0 degrees C) and 2.92 +/- 0.04 (20.0 degrees C) in 1.0 M H+, Na+, ClO4-; Mn(III), 0.22 +/- 0.04 (7.6 degrees C) and 0.08 +/- 0.04 (20.0 degrees C) in similar to 4 M H+, Mn2+, ClO4-. Global fits of the data for each metal have been used to estimate the Delta H degrees and Delta S degrees values for K-h. It is suggested that the unusually high acidity of Mn(OH2) (3+)(6) may be due to a Jahn-Teller distortion effect.