Several solution properties of complexes formed between the trivalent lanthanide ions (Ln(III)) and the macrocyclic ligand DOTP8-, including stability constants, protonation equilibria, and interactions of the LnDOTP(5-) complexes with alkali metal ions, have been examined by spectrophotometry, potentiometry, osmometry, and H-1, P-31, and Na-23 NMR spectroscopy. Spectrophotometric competition experiments between DOTP and arsenate III for complexation with the Ln(III) ions at pH 4 indicate that the thermodynamic stability constants (log K-ML) Of LnDOTP(5-) range from 27.6 to 29.6 from La-III to Lu-III. The value for LaDOTP5- obtained by colorimetry (27.6) was supported by a competition experiment between DOTP and EDTA monitored by H-1 NMR (27.1) and by a potentiometric competition titration between DTPA and DOTP (27.4).