The stability constants of the LnDOTA- complexes have been determined by spectrophotometry at 37 °C in 1.0 M NaCl. The log K values of CeDOTA− and EuDOTA− were obtained by a direct method using the UV absorption of Ce3+ and Eu3+. For the other lanthanides log K values were obtained from the study of the competition reaction between the Ln3+ ion and the Ce3+ or Eu3+ ion for DOTA. The stability constants increase with the decrease in the ionic size between La3+ and Eu3+, while for the heavier lanthanides the log K values are practically constant.