Compounds with the formula (\mathrm{U}{3} M{2}^{\prime} M_{3}^{\prime \prime}) have been synthesized for (M^{\prime}=\mathrm{Al}, \mathrm{Ga}) and (M^{\prime \prime}=\mathrm{Si}, \mathrm{Ge}). The crystal structures of (U_{3} \mathrm{Al}{2} \mathrm{Si}{3}) and (\mathrm{U}{3} \mathrm{Al}{2} \mathrm{Ge}{3}) have been determined from single crystal (X)-ray counter data and were found to be isopointal with the (\mathrm{Cr}{5} \mathrm{~B}{3}) type. Atom order has been refined for (\mathrm{U}{3} \mathrm{Al}{2} \mathrm{Ge}{3}(a=0.77579(13) \mathrm{nm}), (c=1.10357(28) \mathrm{nm})), revealing a random occupation for (8 \mathrm{Ge}+) (8 \mathrm{Al}) in the (16 l) sites and (4 \mathrm{Ge}) in the (4 c) sites (space group (14 /) (m c m-D_{4 h}^{18}), No. 140, (\left.Z=4\right)). For 317 (276) reflections (\left(\left|F_{0}\right|>3 \sigma\right)) the obtained residual values, (R_{\mathrm{x}}=\mathbf{\Sigma}|\Delta F| / \mathbf{\Sigma}\left|F_{0}\right|), were (R_{x}=0.057) for (\mathrm{U}{3}(\mathrm{Al}, \mathrm{Si}){5}) and (R_{\mathrm{x}}=0.074) for (\mathrm{U}{3} \mathrm{Al}{2} \mathrm{Ge}{3}), respectively. Isotypism with the crystal structure of (\mathrm{U}{3} \mathrm{Al}{2} \mathrm{Ge}{3}\left(\mathrm{Cr}{5} \mathrm{~B}{3}\right.)-type derivative) was confirmed from (\mathrm{X})-ray powder diffraction analysis for the novel compounds (\mathrm{U}{3} \mathrm{Ga}{2} \mathrm{Si}{3}) and (\mathrm{U}{3} \mathrm{Ga}{2} \mathrm{Ge}{3}). Whereas the compounds (\mathrm{U}{3} \mathrm{Al}{2} \mathrm{Ge}{3}) and (\mathrm{U}{3} \mathrm{Ga}{2}{\mathrm{Si}, \mathrm{Ge}}{3}) were obtained at the given stoichiometry, a small homogeneous range was observed for (\mathrm{U}{3+\mathrm{y}}\left(\mathrm{Al}{1-x}\right.) (\left.\mathrm{Si}{x}\right){5-y}) ranging in as-cast alloys from (x=0.64) to (x=0.68) and (0 \leq y \leq 0.04 . U_{3}(\mathrm{Al}, \mathrm{Si}){5}) is a high-temperature compound only observed in as-cast alloys and transforms on annealing at temperatures below (\sim 1000^{\circ} \mathrm{C}). For the homologous phases (\mathrm{U}{3} \mathrm{Al}{2} \mathrm{Ge}{3}) and (\mathrm{U}{3} \mathrm{Ga}{2}(\mathrm{Si}, \mathrm{Ge})_{3}) no transformation has been observed in the range from (600^{\circ} \mathrm{C}) to the melting. 1994 Academic Press, Inc.