Two new phases, U(Ta,W) 5 O 16 and U(Ta,W) 2 O 8 , were formed by solid state reaction of the binary oxides (U 3 O 8 , Ta 2 O 5 , and WO 3 ) and Ta metal powder. They were characterized by high-resolution transmission electron microscopy and X-ray powder diffraction techniques. The phases are orthorhombic with the following unit cell dimensions: U(Ta,W) 5 O 16 , a ؍ 10.052(1), b ؍ 7.3207(5), and c ؍ 7.9600(4) A > ; and U(Ta,W) 2 O 8 , a ؍ 20.232(2), b ؍ 7.4520(4), and c ؍ 4.0652(2) A > . Both structures are built up of (Ta,W)O 6 -octahedra arranged as ReO 3 -type slabs two octahedra wide. In U(Ta,W) 5 O 16 these slabs are connected by a thin lamella consisting of hexagonal UO 8 -bipyramids and (Ta,W)O 6 -octahedra alternating along both the b and c axes. EDS analysis indicated a composition of UTa 1.5 W 3.5 O 16 . The structure is related to UMo 5 O 16 , but with every other layer displaced through 1 2 b. The second phase, U(Ta,W) 2 O 8 , is isotypic with the orthorhombic modification of UMo 2 O 8 , in which the ReO 3 -type slabs are intergrown with lamellas of edge-sharing pentagonal UO 7 -bipyramids. The composition UTa 0.5 W 1.5 O 8 was obtained by EDS analysis. Some defects in terms of threeoctahedra-wide ReO 3 -type slabs have been observed in U(Ta,W) 5 O 16 crystals. EDS analyses of such crystals showed a high-tungsten and a low-tantalum content.