The terbium-tris[(2-hydroxy-benzoyl)-2-aminoethyl]amine complex (Tb-THBA) with its high color purity, long luminescence lifetime, and high quantum yield has been found to be a viable indicator for the optical sensing of temperature. Both its luminescence intensity and its lifetime strongly depend on temperature in the range from 15 to 65 degrees C. When photoexcited at 341 nm, it displays typical Tb(3+) ion emission bands with the strongest peak at 546 nm and a typical decay time of 1.15 ms at 15 degrees C. The probe is shown to be excellent for sensing temperature, as demonstrated in two kinds of optical sensor membranes. In the first, it was incorporated into a highly biocompatible polyurethane hydrogel to form a sensing film. In the second, Tb-THBA was converted into nanoparticles with a mean diameter of 10 nm that were then incorporated into a film of poly(vinyl alcohol). The two films display a remarkably high sensitivity toward temperature change, both in luminescence intensity and in luminescence decay time, making them promising for the optical sensing and imaging of temperature in the physiologically relevant temperature range. The mechanism behind the temperature sensing has been investigated using a combination of experimental techniques. For the complex in solution or the polyurethane sensing film, the emission intensity and lifetime decrease with increasing temperature, which is expected and attributed to thermal deactivation of the excited state. For the nanoparticles in solution, however, an interesting and unusual temperature dependence of the emission intensity has been observed. The emission intensity was found to increase with increasing temperature in the range of 20-65 degrees C, which is possibly due to a shift in equilibrium from a less luminescent species or state to a more luminescent species or state. For the nanoparticle films, this unusual behavior disappeared, likely due to the lack of such an equilibrium shift in the films.