A novel class of 1,4,7,10-tetraazacyclododecane-1,4,7-tris(methylenecarboxylic) acid (DO3A)-based lanthanide complexes with relaxometric response to Ca2+ was synthesized, and their physicochemical properties were investigated. Four macrocyclic ligands containing an alkyl-aminobis(methylenephosphonate) side chain for Ca2+-chelation have been studied (alkyl is propyl, butyl, pentyl, and hexyl for L-1, L-2, L-3, and L-4, respectively). Upon addition of Ca2+, the r(1) relaxivity of their Gd3+ complexes decreased up to 61% of the initial value for the best compounds GdL3 and GdL4. The relaxivity of the complexes was concentration dependent (it decreases with increasing concentration). Diffusion NMR studies on the Y3+ analogues evidenced the formation of agglomerates at higher concentrations; the aggregation becomes even more important in the presence of Ca2+. P-31 NMR experiments on EuL1 and EuL4 indicated the coordination of a phosphonate to the Ln(3+) for the ligand with a propyl chain, while phosphonate coordination was not observed for the analogue bearing a hexyl linker. Potentiometric titrations yielded protonation constants of the Gd3+ complexes log K-H1 values for all comlexes lie between 6.12 and 7.11 whereas log K-H2 values are between 4.61 and 5.87. Luminescence emission spectra recorded on the Eu3+ complexes confirmed the coordination of a phosphonate group to the Ln(3+) center in EuL1. Luminescence lifetime measurements showed that Ca-induced agglomeration reduces the,hydration number which is the main cause for the change in r(1), Variable temperature O-17 NMR experiments evidenced high water exchange rates on GdL1, GdL2, and GdL3 comparable to that of the aqua ion.