[NF4Cu(OH)MoO4] as active photocatalyst in the decomposition of Congo Red when irradiated by UV or visible light has been prepared in an unusual ammonia/water ligand exchange reaction of [tetraamminecopper(II)] molybdate, [Cu(NH3)(4)]MoO4. [Cu(NH3)(4)]MoO4 was subjected to moisture of open air at room temperature. Light blue orthorhombic [Cu(NH3)(H2O)(3)]MoO4 was formed in 2 days as a result of an unexpected solid/gas phase ammonia-water ligand exchange reaction. This complex does not lose its last ammonia ligand on further standing in open air; however, a slow quasi-intramolecular (self)-protonation reaction takes place in 2-4 weeks, producing a yellowish-green microcrystalline material, which has been identified as a new compound, [NH4Cu(OH)MoO4], (a = 10,5306 angstrom, b = 6.0871 angstrom, c = 8.0148 angstrom, beta = 64,153 degrees, C2, Z = 4). Mechanisms are proposed for both the sequential ligand exchange and the self-protonation reactions supported by ab initio quantum-chemical calculations and deuteration experiments as well. The [Cu(NH3)(H2O)(3)]MoO4 intermediate transforms into NH4Cu(OH)(H2O)(2)MoO4, which loses two waters and yields [NH4Cu(OH)MoO4]. Upon heating, both [Cu(NH3)(4)]MoO4 and [Cu(NH3)(H2O)(3)]MoO4 decompose, losing three NH3 and three H2O ligands, respectively, and stable [Cu(NH3)MoO4] is formed from both. The latter can partially be hydrated in boiling water into [NH4Cu(OH)MoO4. This compound can also be prepared in pure form by boiling the saturated aqueous solution of [Cu(NH3)(4)]MoO4. All properties of [NH4Cu(OH)MoO4] match those of the active photocatalyst described earlier in the literature under the formulas (NH4)(2)[Cu(MoO4)(2)] and (NH4)(2)Cu(NH3)(3)Mo5O20.