The novel bis(methylphosphonate) 2,2′-[4,10-bis-(phosphonomemyl)-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.