Dilute U-Ga alloys display a variety of metastable structures at room temperature. The addition of 1.5 at.% Ga and rapid cooling from the β-U or the γ-U phase region allow the retention at room temperature of the β-U structure. At the ambient, β-U is metastable and undergoes a shear-like isothermal transformation into α', a distorted α-U-like structure. This transformation is accompanied by severe surface deformation effects. The α'-U structure, with the same composition as the β-U solid solution from which it was formed, decomposes after an aging treatment into a two-phase structure consisting of virtually gallium-free α-U and the intermetallic compound U3Ga5. The higher the gallium content of the initial alloy, the more sluggish the martensitic transformation into the α' structure. At a higher gallium content and slower cooling rates, U3Ga5 Widemanstatten platelets are formed while the adjacent, gallium-depleted matriv reverts to the α-U structure.