Crystals of Cs2Mo4P6O26 and Cs4Mo8P12O52 were both discovered in the same reaction product obtained from heating a stoichiometric mixture of Cs2MoO4, MoO3, MoO2, and P2O5 in an evacuated quartz tube at 800°C. For Cs2Mo4P6O26, monoclinic, P2 c, a = 7.479(3), b = 8.461(5), c = 9.018(2) Å, β = 101.99(3)°, V = 558.2(7) Å3, Z = 1, and R = 0.045, Rw = 0.053 for 936 independent reflections. For Cs4Mo8P12O52, monoclinic, P21, a = 6.398(1), b = 19.497(6), c = 9.835(2) Å, β = 107.06(3)°, V = 1173(1) Å3, Z = 1, and R = 0.046, Rw = 0.058 for 1398 independent reflections. Cs2Mo4P6O26 consists of layers of molybdenum phosphorus oxide with the cesium atoms between the layers. Each layer is built up from corner-sharing MoO6 octahedra, PO4 tetrahedra, and pyrophosphate groups, P2O7. Cs4Mo8P12O52 is composed of the same building units arranged in a different way to give rise to tunnels where the cesium cations reside. The rubidium, potassium, and thallium analogs of Cs4Mo8P12O52 were also prepared. The Cs2Mo4P6O26 structure is closely related to that of K4Mo8P12O52 (A. Leclaire, J. C. Monier, and B. Raveau, J. Solid State Chem. 48, 147 (1983)).