[en] Uranium trioxide is an important intermediate compound in the uranium production process. Inhalation of UO₃ aerosols can occur during this process. To assess the radiation dose from the intake of this compound it is necessary to know its transportability class, based on its dissolution rate in lung fluid. The International Commission on Radiological Protection (ICRP) has assigned UO₃ to Inhalation Class W (lung retention half-time of 10 to 100 days). A solubility study of UO₃ in a simulated lung fluid has been carried out using a batch/filter replacement method. Two tests were conducted over a 100-days period, during which 17 samples were collected and analyzed for their dissolved uranium content. The results show that about 40% of the total uranium was dissolved during the first days and nearly all was dissolved during 100 days. Expressed as the fraction of the total uranium remaining undissolved as a function of time, using a non-linear least squares regression fit, it was found that the solubility of UO₃ in simulated lung fluid could be expressed as a combination of two Inactions: about 25% of the UO₃ could be classified as type D (with lung retention half-time of several hours) and about 75% as type W (with half-time of 10-20 days). This classification is in agreement with recent investigations and indicates that UO₃ is more soluble than considered by ICRP. (authors)