Based on the mixed phthalate (phth(2-)) and adipate (ad(2-)), [Nd-2(ad)(phth)(2)(H2O)(4)] (I) and [Ln(ad)(0.5)(phth)(H2O)(2)] (Ln = Eu-III (II), Gd-III (III), Tb-III (IV), Dy-III (V), Er-III (VI), Tm-III (VII), 1Eu(III):10Tb(III) (VIII), 3Eu(III):10Tb(III) (IX), and 5Eu(III):10Tb(III) (X)) were synthesized and characterized. Complexes VIIIX show excellent ratiometric temperature sensing behavior in physiological and higher temperature ranges (303-423 K) rendered by the Tb-III-to-Eu-III energy transfer process. The efficiency of the process as illustrated through the lifetime measurements depends on both the EuIII:TbIII mole ratio and the temperature. The performance of X in terms of relative sensitivity (S-r), temperature resolution, and measurement repeatability were determined, revealing the maximum S-r (S-m) of 1.21%.K-1 at 303 K with reliable temperature resolution and excellent repeatability.