Oxides of Y, Sm, Eu, Gd, Dy, Ho, and Er were produced by the sol–gel technique. The xerogels obtained as powders after precipitation from salts in water solution were subjected to thermal treatment at constant temperature in the range from 300 to 1300 °C, in air, for 3 h. X-ray powder diffraction analysis of the products was carried out at room temperature. Above a certain threshold temperature all the xerogels turned into pure nanocrystalline oxide phases of cubic Mn₂O₃-type. The grain size of the cubic powders was found to be in the nanocrystalline range, from 10 to 100 nm with accelerating crystal growth as the treatment temperature rose. On increasing of the firing temperatures the oxides of Sm, Eu, and Gd were altered to a monoclinic Sm₂O₃-type structure. The highest firing temperature yielding a cubic structure was 800, 1100 and 1200 °C for Sm, Eu and Gd oxides respectively. On firing at 1300 °C all these oxides were found as a single monoclinic phase. Since the monoclinic phase has higher density (ca. 5% above the cubic) and lower entropy the transition from LT cubic to HT monoclinic is unusual. Another phenomenon was discovered: the cell parameters of Y, and rare earth cubic oxides were significantly dependent on the firing temperature. Higher than normal cell parameter values were found at the lowest firing temperatures, with rapid decreasing upon increasing the firing temperature until reaching asymptotically the bulk cell parameter value. In comparison with other nanostructures [1] both, unusual phase transformations and cell parameter variations can be regarded as grain size effects in the nano range.