A new (\mathrm{Mo}(\mathrm{V})) diphosphate (\mathrm{Cs}(\mathrm{MoO}) \mathrm{P}{2} \mathrm{O}{7}) with a tunnel structure has been iscolated. It erystallizes in the space group (\boldsymbol{P 2}, \mathbf{n}) with (a=5.134 \AA, b=11.707 \AA, c=12.063 \AA, \beta=91.77^{\circ}). The (\left{\mathrm{MoP}{2} \mathrm{O}{8} \mathrm{l}{\infty}\right.) frantework consists of (\left{\mathrm{MoP}{2} \mathrm{O}{11} \mathrm{l}{\infty}\right.) chains waving along b and (\mathrm{c}) and built up of corner-sharing (\mathrm{P}{2} \mathrm{O}{7}) groups and (\mathrm{MoO}{6}) octahedra, like in (\mathrm{K}(\mathrm{MoO}) \mathrm{P}{2} \mathrm{O}{7}). The two structures (\mathrm{K}(\mathrm{MoO}) \mathrm{P}{2} \mathrm{O}{7}) and (\mathrm{Cs}(\mathrm{MoO}) \mathrm{P}{2} \mathrm{O}{7}) differ by the configuration of their (\mathrm{P}{2} \mathrm{O}{7}) groups which are eclipsed and staggered respectively. In fact, this structure is closely related to that of (\alpha-\mathrm{NaTiP}{2} \mathrm{O}{7}). It is built up similarly of layers of diphosphate groups and of (\mathrm{MoO}{6}) octahedra stacked along (b) and forming similar hexagonal tunnels running along ([101]) and {101}. This structure differs from that of (\alpha-\mathrm{NaTiP}{2} \mathrm{O}{7}) by the staggered configuration of its (\mathrm{P}{2} \mathrm{O}{7}) groups leading to different orientation of the (\mathrm{MoO}{6}) octahedra between two successive octahedral layers, and by the fact that some (\mathrm{P}{2} \mathrm{O}{7}) groups and (\mathrm{MoO}{6}) are disconnected. O 1994 Academic Press. Inc.