Lanthanum oxide compounds are of great interest because of their wide range of applications (e.g., heterogeneous catalysis). It is well known that lanthanum oxide compounds, like lanthanum oxide, have to be handled carefully in a humid atmosphere due to their hydration affinity. Furthermore, lanthanum hydroxide has a high CO2 affinity and partially forms basic carbonates. The amount of carbonate impurities that are formed in air remains nearly constant over a long time. To investigate the carbonation process in more detail, lanthanum hydroxide and lanthanum oxide were stored in a controlled humid CO2 atmosphere. The results of thermal analysis (TA) and X-ray diffraction (XRD) prove a transformation of lanthanum hydroxide as well as lanthanum oxide to lanthanum carbonate via lanthanum hydroxide carbonate.