The exploration of A–Mo–P–O (A = Rb, Cs) systems has allowed several new Mo(V) phosphates, Rb₂MoP₂O₉, Rb₆Mo₂P₄O₁₉ and Cs₆Mo₂P₄O₁₉, to be synthesized through the spontaneous nucleation method. Single-crystal X-ray diffraction analysis reveals that the identical stoichiometry compounds Rb₂MoP₂O₉ and Cs₂MoP₂O₉ belong to different space groups C2/c and Pbca, respectively. Both compounds consist of dissimilar 1D [Mo–O–P]_∞ chains with different repeated building units, while monovalent cations fill in spaces to form 3D structures. However, Rb₆Mo₂P₄O₁₉ and Cs₆Mo₂P₄O₁₉ are isostructural and crystallize in the same space group of P2₁/c. They exhibit a 3D framework structure with 0D Mo₂O₅O₆P₄O₈ groups, which are separated by Rb/Cs atoms. Interestingly, structural relationships between the different monophosphates of the A–Mo–P–O (A = Rb, Cs) systems are presented in which distinct polyanionic configurations appear owing to the A/P ratios, as well as the size of the univalent cations. Further, detailed structural comparisons, optical properties and theoretical calculations are also discussed.