In this study, the kinetic behaviors of four lanthanide ions, Sm3+, Dy3+, Yb3+, and Lu3+ when mixed with a polyazamacrocyclic chelating agent 1,4,7,10-tetraazacyclododecane-N,N',N",N--tetraacetic acid (DOTA), were investigated by capillary electrophoresis (CE) in the pH range of 2.0-6.0. At pH 2.0, the formation rate of DOTA-metal complex is extremely low as very little complex was detected after 5 days reaction, whereas almost no free DOTA was found in the mixture of metal ion and DOTA after 4 min at pH 6.0. The second-order kinetic association rate constants of the four lanthanide ions chelates at pH 4.2 were calculated as 1.44 x 10(-2) mm(-1)min(-1), 5.20 x 10(-2) mm(-1)min(-1), 4.56 x 10(-2) mm(-1) min(-1) and 4.54 x 10(-2) mm(-1)min(-1) at 25degreesC with CE, respectively. In addition, the stability constants of the four lanthanide ions with DOTA were determined by CE at pH 3.0 where approximately 80-90% of the metal ions were associated with DOTA at 25degreesC. The measured stability constants (log K-t) of the four DOTA-metal complexes were 23.36, 23,93, 23.39 and 23.06, respectively, and correlated well with published data obtained by different methods. The percentage of metal ion bound with DOTA was evaluated as a function of reactant concentration in pH 6.0 buffer, After adding excess strong acid (0.1 m HCl) to each solution of DOTA-metal was formed at pH 6.0, no released DOTA was detected after 24 h; thus, dissociation of these lanthanide complexes did not occur under strongly acidic conditions. The Ln(DOTA)(-) species for the four DOTA-metal complexes were characterized by electrospray ionization-mass spectroscopy (ESI-MS), and the results correlated with proposed structures.