Comprehensive studies on the radiotoxicological risk of an intermediate compound U04, which is not specified in ICRP Recommendations, were motivated by its increased use in the nuclear fuel cycle and the lack of information such as physico-chemical and biokinetic properties. The aim of this work was to give an experimental basis for assessing the appropriate limits on intake for workers exposed to U04 and to provide guidance for the interpretation of personal monitoring data. Particle size measurement of the U04 dust indicated a geometric diameter D of 0.5 um, which corresponds to an activity median aerodynamic diameter (AMAD) of 1.1 um. In vitro experiments conducted in culture medium showed that U04 is a soluble compound with 66.2% dissolved in 1.9 d and 33.8% in 78 d. Results of dissolution obtained with macrophages showed a significant decrease of 50% at 1 d in terms of solubility. Biokinetic data in the rat obtained from two in vivo studies involving intratracheal instillation in rats indicated half-times in the lung of 0.5 d (96.6%) and 27 d (3.4%) for an initial lung deposit (ILD) of 195 ug, and 1.2 d (90.3 %) and 38 d (9.7%) for an ILD of 7.6 fig. Absorption parameters to blood as defined in the ICRP Publication 66 human respiratory tract model were calculated with the specific software GIGAFIT and led to the rapid fraction fr (0.800 to 0.873), the rapid rate sr (0.525 to 0.928 d−1), and the slow rate ss (1.57 × 10−2 to 2.42 × 10−3 d−1). Effective dose coefficients by inhalation for this U04 compound using the in vivo experimental results were calculated to be between 0.52 and 0.70 × 10−6 Sv Bq−1. Comparison of these values with effective dose coefficients defined in ICRP Publication 68 for workers showed that U04 could be considered as a fast soluble compound of Type F.