The interaction between the zwitterionic buffers (3-[N-bis(2-hydroxyethyl)amino]-2-hydroxy propane sulfonic acid, N-(2-actamido)-2-aminoethane sulfonic acid, and 3-[(1,1-dimethyl-2-hydroxyethyl)amino]-2-hydroxy propane sulfonic acid) with some divalent transition metal ions (Cu-II, Ni-II, Co-II, Zn-II, and Mn-II) were studied at different temperatures (298.15 to 328.15) K at ionic strength I= 0.1 mol (.) dm(-3) NaNO3 and in the presence of 10%, 30%, and 50%,) (w/w) dioxene by using potentiometry. The thermodynamic stability constants were calculated as well as the free energy change for the 1: 1 binary complexation. The protonation constants of the zwitterionic buffers were also determined potentiometrically under the above conditions. (c) 2006 Elsevier Ltd. All rights reserved.