Three novel polyoxomolybdates with peroxo ligands, Rb₅[Na(H₂O)₅][Mo₇(O₂)₂O₂₂]∙H₂O (Rb-1), Cs₅[Na(H₂O)₅][Mo₇(O₂)O₂₃]∙H₂O (Cs-1) and Cs₄[Mo₄(O₂)₂O₁₂]∙H₂O (Cs-2) have been synthesized through a slow evaporation method. The title compounds were characterized using powder X-ray diffraction and single crystal X-ray diffraction. All compounds crystallize in the triclinic space group, P-1. Both Rb-1 and Cs-1 consist of Rb⁺ or Cs⁺cations, Na(H₂O)₅ and polyoxyanion clusters, [Mo₇(O₂)₂O₂₂]^6- (Figure 1) and [Mo₇(O₂)O₂₃]^6- (Figure 2), while Cs-2 consists of Cs⁺ cations and [Mo₄(O₂)₂O₁₂]^4- (Figure 3) polyoxyanions. All the reported compounds contain peroxo ligand and the oxidation states of the molybdenum are +6. An interesting phase transition from Cs-1 to Cs-2 was monitored during the evaporation of Cs-containing solutions. It is considered that Cs-1 is a metastable compound, whereas Cs-2 is thermodynamically stable product. Further studies to elucidate the transition mechanism of the reported compounds are ongoing.