Ceria (CeO2) crystalline compound is widely used as a catalyst or catalyst support and many other applications. However, the studies are continuing with a view to improving existing methods and/or developing innovative routes to obtain well -controlled shapes and sizes of ceria nanoparticles. In the present study, we report an original method to synthesize ceria nanoparticles (CeO2 and CeO2-X) by using two independent stages: (1) the precipitation of bastnasite-rich material under hydrothermal conditions (90 and 300 degrees C) and (2) the calcination of powdered basnasite-rich material at different temperatures (500, 1000 and 1600 degrees C) and under different atmospheres (air, Ar, N-2 and secondary vacuum). In addition, simultaneous thermal analyses (TGA/DSC) coupled with gas chromatography (mu GC/MS) were performed in order to investigate on the ceria formation during bastnasite de -carbonation and its thermal behavior at high temperature (until 1600 degrees C) under three different gas atmospheres (air, Ar and N-2). Herein, ceria was in -situ formed independently on the gas investigated atmosphere. This means that sufficient oxygen was also available in the so-called inert atmospheres (Ar and N-2) to oxidize the Ce(III) contained in Ce-carbonates to Ce(IV) constituting ceria cubic structure. Moreover, mu GC/MS measurements confirm that in-situ formed ceria CeO2 at about 600 degrees C is then partially reduced to CeO2-x at high temperature after 1000 degrees C because an increase in oxygen was clearly detected in expelled gas during heating process. This fundamental experimental study provides the hydrothermal conditions to which Ce fluorocarbonates could be formed in natural environments. In addition, new experimental conditions to produce ceria with oxygen vacancies CeO2-X without reducing gas agent and at lower temperature are also provided in this study. (C) 2016 Elsevier B.V. All rights reserved.