A ratiometric thermometer based on a mixed Eu–Tb metal–organic framework is reported with a maximum relative thermal sensitivity of 16.1% K⁻¹ at 11 K making this material one of the best-performing thermometers currently reported in the literature. The sensing mechanism of {[Tb_2−xEu_x(H₂Btec)(Btec)(H₂O)₂]·4H₂O}_n (Btec = 1,2,4,5 Benzenetetracarboxylic acid) has been attributed to a phonon-assisted energy migration between neighboring Tb³⁺ ions in the structure.