UV-Vis and lanthanide-induced 17O shift measurements on the complex [Eu(DOTA)]– (H4DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) have shown that the inner co-ordination sphere of the Ln3+ ion is not affected on protonation which suggests that the proton is attached to a non-co-ordinated oxygen atom of a carboxylate group. Proton NMR measurements performed as a function of the H+ concentration revealed that the protonation slightly accelerates the intramolecular dynamic processes: the enantiomerization for [La(DOTA)]– and the enantiomerization and interconversion between the major and minor isomer for [Eu(DOTA)]–. Contrary to first glance expectations, the water exchange rate on [Gd(DOTA)(H2O)]– decreases significantly with increasing extent of protonation, and at 1.0 M H+ concentration is about ten times lower than in neutral media. In 1.0 M acidic solution the proton relaxivities were found to be higher than expected solely on the basis of the water exchange rates. This finding is interpreted with a faster proton exchange in acidic solutions which is the consequence of the catalytic effect of H+ ions.