Development of the responsive probes is one of the most exciting topics in the contrast agent research. Design of the pH sensitive probes, for example, can be an important issue in the early cancer diagnostic and formation of pH maps.[1] This can be reached by introduction of different functional groups such as phosphonates at the fourth nitrogen of the DO3A molecule. [2] Two DO3A-based ligands and their Gd3+ and Eu3+ complexes containing ethyl-protected and unprotected propylphosphonates in the side chain were investigated. Proton relaxometric in vitro studies at 20 MHz and 60 MHz and 37 °C of the Gd3+ complex containing free acid exhibited relative changes of up to 56% in r1 relaxivity when the pH of the medium was changed from 4 to 7. This change is explained by the increase in the number of coordinated water molecules from 1 to 2. Temperature dependent relaxivity and