This work intends to investigate samarium and holmium complexes with a set of H4dota-analogues bearing methylphosphonate/methylphosphinic acid pendant arms (H5do3ap, H5do3apPrA and H4do3apABn) as new bifunctional ligands for radiolanthanide targeted therapy. Radiolanthanide complexes of these ligands were prepared and evaluated and thermodynamic solution studies were also performed to get a better insight on the radiochemical results. The radiocomplexes were quantitatively prepared, are hydrophilic and negatively charged. They present a high in vitro and in vivo stability, a low plasma protein binding and a rapid clearance from the main organs with a high rate of whole body radioactivity excretion. Stability constants with Sm(III) and Ho(III) were determined by potentiometry and the values found are very high for all the ligands. Actually, they are similar to those of H4dota for the phosphinate analogues and even higher for the phosphonate ligand, being the constants similar to those with other lanthanide(III) ions. The effect of methylphosphorus acid pendant arms functionalization and the use of these ligands as bifunctional chelate agents are discussed. Copyright © 2009 John Wiley & Sons, Ltd.