Thin-Film Calorimetry (TFC) as presented in this work is a novel analytical tool to determine phase transformation temperatures and enthalpies of thin films and thin-film sequences. The key component is a high-temperature stable piezoelectric langasite (La3Ga5SiO14) resonator serving as a highly sensitive planar temperature sensor. Deviations in its frequency are related to temperature fluctuations caused by phase transformations and used to calculate the related enthalpies. Temperature ramps from room temperature up to 1000°C are applied to perform calorimetric thin-film investigations. Thereby, the atmosphere can be controlled. To the best of our knowledge, the presented TFC is the only existing technique combining the aspects “thin films” and “high-temperature calorimetry.” The first part of this article describes the newly developed TFC system. The second part presents TFC investigations on lithium manganese oxide (LMO) thin films. Measurements are carried out in ambient air and in 0.5 %H2/Ar. In air three phase transformations appear (at 330, 410 and 600°C) while in 0.5 %H2/Ar four phase transformations are observed (at 389, 471, 730 and 758°C). Their progression and related enthalpies are discussed. To determine the associated crystallographic phases, X-ray diffraction and Raman spectroscopy are performed.