The porous structure of intermediates in the thermolysis of hexahydrated uranyl nitrate was found to provide an interesting case study for the use of nitrogen adsorption-desorption isotherms : nonrigid sheetlike pore structures, rigid mesopores, interconnected structures, and clearly bimodal pore size distributions were the situations encountered. The use of sample controlled thermal analysis (SCTA) made it possible to compare two highly reproducible and well-defined thermolysis routes (under residual pressures of 10(-1) and 20 mbar, respectively) leading to very different porous UO3. Adsorption studies, with the help of SEM. investigations and prior X-ray studies, permit a proposition for the mechanism of the formation of the different porous structures.