The stability of the gamma phase in cast U–9wt.% Mo alloy with respect to different thermo-mechanical treatments was investigated. The bcc allotrope of U, desirable due to its irradiation withstanding characteristics, could be retained in a metastable state in this alloy even without rapid cooling from the high temperature gamma phase. The presence of 9wt.% of Mo in the gamma-U phase together with a homogenization annealing heat treatment at 1000°C for 24h increased the stability of the gamma phase against eutectoid decomposition into alpha-U and γ′ (U₂Mo) phases. The mechanism of eutectoid decomposition, engendering different types of microstructure, and the crystallographic orientation relationships, amongst constituent phases, were established through transmission electron microscopy.