Treatment of MoO3 with butan‐1‐amine (BuNH2) or pyrrolidine (PyrNH2) results in the formation of (BuNH3)8[(Mo7O24)(MoO4)]·3H2O (1; BuNH3 = butan‐1‐aminium) or (PyrNH2)6[(Mo7O24]·2H2O (2; PyrNH2 = pyrrolidinium), respectively. On irradiation with sunlight, compound 1, which is an organic heptamolybdate [Mo7O24]6– that contains a cocrystallized monomolybdate [MoO4]2– in the same compound, is transformed to an oxido‐bridged diheptamolybdate (BuNH3)10[(Mo7O22)(μ2‐O)2(Mo7O22)]·5.5H2O (1b). The [Mo7O24]6– units in 1 and 2 are made up of edge‐sharing [MoO6] octahedra, whereas the central unit in 1b is a bis(μ2‐O)‐bridged heptamolybdate dimer. Hydrogen bonding among the lattice water molecules in 1 results in the formation of a water octamer, whereas hydrogen bonding between the lattice water molecules and [Mo7O24]6– in 2 results in the formation of a one‐dimensional water‐linked heptamolybdate chain. Supramolecular structures, photochemistry, and thermal properties of 1, 1b, and 2 are reported. For comparison of photochemical behavior, two more polyoxomolybdates, (PrNH3)6[Mo7O24]·3H2O (3; PrNH3 = propan‐1‐aminium) and (PentNH3)6[Mo7O24]·3H2O (4; PentNH3 = pentan‐1‐aminium), were prepared and characterized.