The acidity constants of 1,4,7-triazacyclononane hydrochloride {(H-3-[9]-ane-N-3)(3+). 3Cl(-)} at different temperatures and the stability constants of its chelates with Mn(II), Fe(II), Fe(III), Co(II), Co(III), and Cu(II) have been determined pH-metrically in aqueous solution in the presence of 0.1 M KN0(3) at 20 degrees C and 32 degrees C. The mole fractions of different species were calculated and discussed. The thermodynamic parameters for the complexes under investigation were also evaluated.