The influence of pressure and temperature on Sm 2O 3 hydration, under mild conditions, are examined. The data and conclusions are referred to previous ones for Yb 2O 3. At the activation temperatures usually applied, as reported in the literature, to rare earth sesquioxides handled in air, 800–850 K, a non-pure-oxide phase is attained in the case of Sm 2O 3. Its complete dehydration is achieved at 823 K, but temperature programmed decomposition and Fourier transform-IR data lend support to the formation at this temperature of a dioxymonocarbonate phase which is transformed into the sesquioxide phase above 1100 K. Contrary to the finding for Yb 2O 3, hydration in bulk Sm 2O 3 has been observed over the whole pressure range 133–2666 Pa. In the absence of CO 2, the temperature of evolution of H 2O from bulk-hydrated Sm 2O 3, about 600 K, is considerably higher than that previously reported for Yb 2O 3, 440 K, following the cation polarizing pattern for the decomposition of metal hydroxides and oxosalts.