Abstract Four different uranium powders were studied by mercury porosimetry. Two powders are initial untreated powders after the conversion. Two others are granulated. From the volume of penetrated mercury in the grains, and the volume expelled from the pores as a function of increasing or decreasing pressure, it is possible to characterize the powders: the type of hysteresis when the mercury is expelled, gives indication on the pores of the grains. The difference between the two expelled volumes during the first penetration of mercury in the powder gives the intra-agglomerate and inter-agglomerate volumes. An agglomeration coefficient has been calculated for the different powders. The mercury volume of the second penetration permits to indicate the quality of the grains: dense or porous.