Nenhuma / Phase transformations in U-Mo alloys, in the as-cast condition, with Mo additions of 5, 7 to 10 wt.%, were investigated by performing artificial aging heat treatments at temperatures of 300 to 500 C. The use of scanning electronic microscopy with field emission gun and electrostatic force microscopy, in combination with conventional microstructural characterization techniques, allowed the detailed description of the microstructure in terms of the constitution and morphology of the transformation product phases which had not yet been reported for U-Mo alloys. The dendritic structure of the as-cast condition was characterized by the presence of molybdenum segregation, which was evidenced by the variation of the Mo content around the nominal composition of the alloys, between the centre and boundaries of the dendrites. The influence of Mo segregation was observed after the artificial ageing heat treatments carried out at both temperatures. Segregated regions with Mo contents below 6.3% presented twinned microstructures, associated with diffuse X-ray diffraction patterns and a remarkable hardening related to disorder-order transformation type, after aging at 300 oC. When submitted to ageing at 500 C, such Mo contents lead to a complete decomposition of the phase or its o variant into a non-lamellar constituent composed by the gamma and alpha phases with microhardness above 580 HV. The heterogeneous nucleation of a lamellar constituent, whose morphology is typical of the divergent discontinuous precipitation mechanism, reported for U-Nb alloys, was responsible for the consumption of the non-lamellar constituent with the extension of the ageing heat treatment. Mo contents between 6.3 and 8.0% determined the absence of twinned morphology after ageing at 300 C, showing a hardening effect only for the 240 hours treatment, as a result of the disorder-order reaction. The cellular decomposition of the phase in these regions took place after ageing at 500 C. A two-phase and alpha e gamma constituent with lamellar morphology and a typical hardness of 450 HV was observed. In the case of the U-10Mo alloy, three transformation mechanisms were observed after ageing at 500 C, as a result of Mo segregation. The lamellar constituent was found in the dendrites boundaries whose Mo contents ranged between 7.4 and 8.6%. For the intermediate portions between the center and the dendrites boundaries, known as interdendritic region, where Mo contents between 8.6 and 10.2% were found, there was no decomposition of the gamma phase in the early stages of aging while the precipitation of the gamma phase, with acicular or Widmanstätten morphology, was noticed in the central portion of the dendrites, with Mo contents ranging from 10.2 to 12.0%. This observation is an evidence that the ability to stabilize the phase provided by the Mo, passes through a maximum and corresponds to the eutectoid composition of U-Mo system, unlike previous reports in the literature which relate a delay in the onset of phase decomposition even for Mo contents higher than 10%.