LaFeOxNy thin films have been deposited by magnetron sputtering in Ar/O2/N2 gas mixture at 800 °C. Such oxynitride perovskites present a specific infrared vibration mode at 2040 cm−1, due to the presence of nitrogen, which disappears during heating in air. The loss of the vibration mode with temperature has been monitored allowing the determination of an activation energy of thermal degradation of LaFeOxNy. The quantification of nitrogen by Elastic Recoil Detection Analysis (ERDA) before and after heating exhibits the same nitrogen content. Such behavior is due to a nitrogen redistribution observed by Electron Energy Loss Spectroscopy (EELS) mapping, showing migration of nitrogen into grain boundaries, in association with the film oxidation.