Nanocrystallites of UO(2) with a size of 3-5 nm were studied in situ with high temperature X-ray diffraction (HT-XRD), thermogravimetry (TGA), and differential thermal analysis. The evolution of the crystallite size, the lattice parameter, and the strain were determined from ambient temperature up to 1200 A degrees C. Below 700 A degrees C, a weak effect on the crystallite size occurs and it remains below 10 nm, while a strong expansion of the lattice parameter is measured. The strain decreases with temperature and is completely released at 700 A degrees C. Above this temperature, begins the sintering of the nanocrystallites reaching a size of about 80 nm at 1200 A degrees C. The weight loss curve observed in TGA is assigned to the desorption of water molecules and is correlated with the strain evolution observed by HT-XRD. The linear thermal expansion and the thermal expansion coefficient at 800 A degrees C are 1.3% and 16.9 x 10(-6) A degrees C(-1), respectively.