A new molybdenum ( (\mathrm{V}) ) monophosphate (\mathrm{K}{2} \mathrm{Mo}{2} \mathrm{P}{2} \mathrm{O}{11}) has been isolated. The single crystal (\mathrm{X})-ray diffraction study of this phase has shown that it is isotypic with (\mathrm{K}{2} \mathrm{Nb}{2} \mathrm{As}{2} \mathrm{O}{11}). It crystallizes in the (P 21 / c) space group with (a=9.867) (2), (b=10.122(1), c=9.903) (2) (\AA, \beta=97.95(1)^{\circ}). A new description of the struclure is given, based on the existence of (\mathrm{Mo}{2} \mathrm{P}{2} \mathrm{O}{15}) units formed of one (\mathrm{MO}{2} \mathrm{O}{13}) group sharing four of its corners with two (\mathrm{PO}{4}) letrahedra and leading to (\left[\mathrm{Mo}{2} \mathrm{P}{2} \mathrm{O}{13}\right]{x}) chains running along (\mathrm{c}). The (\left[\mathrm{Mo}{2} \mathrm{P}{2} \mathrm{O}{11}\right]{1 \mathrm{x}}) framework forms two different intersecting tunnels running along (\mathrm{c}) and (\langle 1 \mathrm{I} 0\rangle), respectively, where the (\mathrm{K}^{+})cations are locited. The existence of one free apex for each (\mathrm{MoO}_{6}) octahedron, which characterizes Mo(V), allows a great flexibility of the structure. O 1993 Academic Press. Inc.