In this paper, with the example of two different polymorphs of KEu(MoO 4) 2 , the influence of the ordering of the A-cations on the luminescent properties in scheelite related compounds (A′,A″) n [(B′,B″)O 4 ] m is investigated. The polymorphs were synthesized using a solid state method. The study confirmed the existence of only two polymorphic forms at annealing temperature range 923−1203 K and ambient pressure: a low temperature anorthic α-phase and a monoclinic high temperature β-phase with an incommensurately modulated structure. The structures of both polymorphs were solved using transmission electron microscopy and refined from synchrotron powder X-ray diffraction data. The monoclinic β-KEu(MoO 4) 2 has a (3+1)-dimensional incommensurately modulated structure (superspace group I2/b(αβ0)00, a = 5.52645(4) Å, b = 5.28277(4) Å, c = 11.73797(8) Å, γ = 91.2189(4) o , q = 0.56821(2)a*−0.12388(3)b*), whereas the anorthic α-phase is (3+1)-dimensional commensurately modulated (superspace group I1̅ (αβγ)0, a = 5.58727(22) Å, b = 5.29188(18)Å, c = 11.7120(4) Å, α = 90.485(3) o , β = 88.074(3) o , γ = 91.0270(23) o , q = 1/2a* + 1/2c*). In both cases the modulation arises due to Eu/K cation ordering at the A site: the formation of a 2-dimensional Eu 3+ network is characteristic for the α-phase, while a 3-dimensional Eu 3+-framework is observed for the β-phase structure. The luminescent properties of KEu(MoO 4) 2 samples prepared under different annealing conditions were measured, and the relation between their optical properties and their structures is discussed.