Protonation constants of 1,3-bis(tris(hydroxymethyl)methylamino)propane (BTP) and the complexes formed between BTP and Co(II), Ni(II), or Zn(II) metal ions have been studied by glass electrode potentiometry (GEP) in aqueous 0.1 mol.L(-1) KNO(3) supporting electrolyte at 25 +/- 0.1 degrees C. The complexation model for each M(II)-BTP system was established. The proposed final model for Ni(II)-BTP system is: NiL, NiLH(-1), and NiLH(-2), with stability constant values, as log beta, of 7.13 +/- 0.02, -1.32 +/- 0.04, and -10.81 +/- 0.05, respectively. For Co(II)-BTP and Zn(II)-BTP systems, the proposed final model is constituted by ML, ML(2), ML(2)H(-1), and ML(2)H(-2) complexes. For the Co(II)-BTP system, the overall refined stability constants (as log beta) are 4.57 +/- 0.02, 8.76 +/- 0.02, 0.17 +/- 0.03, and -8.73 +/- 0.03, respectively, and for Zn(II)-BTP system they are 4.99 +/- 0.02, 9.18 +/- 0.03, 1.01 +/- 0.03, and -7.91 +/- 0.04, respectively. In a general way, the complex stability was found to follow the trend: Ni(II) > Zn(II) > Co(II).