The novel bis(methylphosphonate) 2,2′-[4,10-bis(phosphonomethyl)-1,4,7,10-tetraazacyclododecane-1,7-diyl] diacetic acid, trans-H₆DO2A2P, has been synthesized and characterized by multinuclear NMR spectroscopy (¹H, ¹³C and ³¹P). ¹⁵³Sm and ¹⁶⁶Ho complexes with trans-H₆DO2A2P have been prepared in high yield using a 1:2 metal to ligand molar ratio at 70 °C, pH 8−9. These complexes are hydrophilic, negatively charged and stable in vitro under physiological solutions, up to 48 h. They present a low plasmatic protein binding and some in vitro hydroxyapatite adsorption, mainly for ¹⁶⁶Ho-trans-DO2A2P. Both complexes are stable in vivo, have a fast tissue clearance from most organs and a rapid total excretion from whole animal body. Moderate bone uptake was also observed but the accumulated radioactivity rapidly decreases with time.