The thermodynamic stability constants of complexes of 1,4,8,11-tetraazacyclotetradecane-1,4,8-triacetic-11-methylphosphonic acid (H5te3a1p) with La3+, Sm3+, Gd3+, Ho3+ and Lu3+ metal ions were determined by potentiometric titrations at 298.2 K and with ionic strength 0.10 M in N(CH3)4NO3. The complexes are formed relatively fast and the stability constants exhibited are good although lower than those found for the related ligands H4teta and H8tetp. At physiological pH the completely deprotonated complex species predominate, unlike what happens with the other mentioned ligands. The 153Sm and 166Ho complexes, 153Sm/166Ho-te3a1p, were synthesised quantitatively at pH 9 and 70 °C, and have shown good in vitro stability in human serum and physiological solutions except phosphate buffer (pH 7.4). The in vivo behaviour indicated that both complexes have a similar biological pattern, showing a slow tissue clearance, slow rate of total radioactivity excretion and some in vivo instability, although with some differences in their extend. These results indicate that the replacement of one acetate pendant arm of H4teta by a methylphosphonate one does not provide promising chelators to stabilize radiolanthanides for in vivo application.