The hydrothermal syntheses and crystal structures of two new alklali-metal molybdenum(VI) methylphosphonates, CS2(MoO3)(3)PO3CH3 and Rb-2(MoO3)(3)PO3CH3, are described. These isostructural phases are built up from hexagonal tungsten oxide like layers of vertex-sharing MoO6 octahedra, capped on one side by P-CH3 entities (as [PO3CH3](2-)methylphosphonate groups). Interlayer Cs+ or Rb+ cations provide charge balance for the anionic sheets. The MoO6 octahedra display a distinctive three short + three long Mo-O bond distance distribution within the MoO6 unit. Powder X-ray, infrared, Raman, and thermogravimetric data for CS2(MoO3)(3)PO3CH3 are also presented. The structures of Cs-2(MoO3)(3)PO3CH3 and Rb-2(MoO3)(3)PO3CH3 are related to those of the recently reported M(2)(MoO3)(3)SeO3 (M = NH4, Cs) materials but show a three-layer repeating motif, rather that the two-layer repeat found in the M(2)(MoO3)(3)SeO3 structures. This different layer repeat pattern may be correlated with the steric effect of the methylphosphonate group. Crystal data : Cs-2(MoO3)(3)PO3CH3, trigonal, R3 (No. 146), a = 7.304(2) Angstrom, c = 20.02(1) Angstrom, V = 924.7 Angstrom(3), Z = 3; Rb-2(MoO3)(3)PO3CH3, trigonal, R3 (No. 146), a = 7.307(2) Angstrom, c = 20.040(4) Angstrom, V = 926.7 Angstrom(3), Z = 3.