AbstractThe rich phase diagram of bulk Pr_1−xCa_xMnO₃ resulting in a high tunability of physical properties gives rise to various studies related to fundamental research as well as prospective applications of the material. Importantly, as a consequence of strong correlation effects, electronic and lattice degrees of freedom are vigorously coupled. Hence, it is debatable whether such bulk phase diagrams can be transferred to inherently strained epitaxial thin films. In this paper, the structural orthorhombic to pseudo‐cubic transition for x = 0.1 is studied in ion‐beam sputtered thin films and differences to the respective bulk system are pointed out by employing in situ heating nano‐beam electron diffraction to follow the temperature dependence of lattice constants. In addition, it is demonstrated that controlling the environment during heating, that is, preventing oxygen loss, is crucial in order to avoid irreversible structural changes, which is expected to be a general problem of compounds containing volatile elements under non‐equilibrium conditions.