Middle-molecular-weight MRI contrast agents based on conjugates of a phosphinic acid DOTA analogue, 1,4,7,10-tetraazacyclododecane-4,7,10-triacetic-1-{methyl[(4-aminophenyl) methyl] phosphinic acid} (DO3AP(ABn)), with amino-substituted cyclodextrins were prepared and studied by a variety of physicochemical methods. The conjugates were formed by reaction of the corresponding isothiocyanate with per-6-amino-alpha/beta-cyclodextrin and were complexed with the Ln(III) ion to get the final complexes, (LnL)(6)-alpha-CD and (LnL)(7)-beta-CD. Solution structure of the complexes was estimated by investigation of the Eu-III complexes. The Gd-III conjugate complexes are endowed with a short water residence time (tau(M) similar to 10-15 ns at 298 K) and a high abundance of the twisted-square antiprismatic diastereoisomer. They show a high H-1 relaxivity at high fields due to a convenient combination of the fast water exchange rate and the slow rate of the molecular tumbling given by their macromolecular nature. The 1H relaxation enhancements per molecule of a contrast agent (CA) are very high reaching for a larger (GdL)(7)-beta-CD conjugate similar to 140 s(-1) mM(-1) and similar to 100 s(-1) mM(-1) at 25 degrees C and magnetic fields 1.5 T and 3 T, respectively, which is the highest reported longitudinal relaxivity for kinetically stable contrast agents of an intermediate molecular mass (<10 kDa) with one water molecule in the first coordination sphere.