(Mo, (\mathrm{Nb}){13} \mathrm{O}{33}) crystallizes in the triclinic system, space group (P 1) (No. 2). The unit cell dimensions are (a=3.8194(4) \AA, b=) (11.2898(7) \AA, c=14.9764(9) \AA, \alpha=94.039(5)^{\circ}, \beta=97.326(6)^{\circ}), (\gamma=99.748(7)^{\circ}, V=628.47(9) \AA^{3}), and (Z=1). The structure was refined to (R=0.032, R_{\mathrm{w}}=\mathbf{0 . 0 3 6}), with 1460 observed reflections ((I>3 \sigma(I))) and 186 variable parameters. The structure of (Mo, (\mathrm{Nb}){1,3} \mathrm{O}{33}) is related to the crystallographic shear structure of the (\mathrm{Nb}{2} \mathrm{O}{5-\delta}) family and is built up from the (3 \times 4 \times \infty) building blocks of the (\mathrm{ReO}{3})-lype (M \mathrm{O}{6}), octahedra ( (M=\mathrm{Mo}, \mathrm{Nb}) ) and strings of edge-sharing (\mathrm{MoO}{4}) tetrahedra ruming along [101|. The (3 \times 4) (\mathrm{ReO}{3})-type slabs at different heights along [ 101] are interconnected through corner sharing with the (\mathrm{MoO}{4}) tetrahedra. (Mo, (\left.\mathrm{Nb}\right){13} \mathrm{O}_{33}) is an insulator at room temperature. 1994 Acadcmic Press, Inc.