Solution properties of complexes of a new H4dota-like ligand containing three acetate and one methylphosphonate pendant arms (H5do3ap, H5L) were studied. The ligand exhibits a high last dissociation constant (pKA = 13.83) as a consequence of the presence of phosphonate moiety. In solution, successive attachment of protons leads to several reorganizations of protonation sites and the neutral zwitterionic species H5do3ap has the same solution structure as in the solid state where the nitrogen atom binding methylphosphonate and the opposite nitrogen atoms are protonated. Stability constants with Na+ and trivalent lanthanide ions (La3+, Ce3+, Eu3+, Gd3+, Lu3+) and Y3+ were determined. The constants are comparable or higher than those of H4dota due to the higher overall basicity of H5do3ap. Formation of the stable protonated complexes as well as complexes with the L:M = 1:2 stoichiometry was proved. Formation and decomplexation kinetics of the Ce3+ and Gd3+ complexes were investigated.