The R2(MoO4)3 (R = rare earth elements) molybdates doped with Eu3+ cations are interesting red-emitting materials for display and solid state lighting applications. The structure and luminescent properties of the R2−xEux(MoO4)3 (R=Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (α-) and orthorhombic (β´-) R2−xEux(MoO4)3 (R=Gd, Sm, 0≤x≤2) modifications were prepared by solid-state reaction and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic β´-phases could be synthesized only by quenching from high temperature to room temperature for Gd2−xEux(MoO4)3 in the Eu3+-rich part (x > 1) and for all Sm2−xEux(MoO4)3 solid solutions. The transformation from the α-phase to the β´-phase results in a notable increase (~24%) of the unit cell volume for all R2−xEux(MoO4)3 (R = Sm, Gd) solid solutions. The luminescent properties of all R2−xEux(MoO4)3 (R = Gd, Sm; 0≤x≤2) solid solutions were measured, and their optical properties were related to their structural properties. All R2−xEux(MoO4)3 (R= Gd, Sm; 0≤x≤2) phosphors emit intense red light dominated by the 5D0 – 7F2 transition at ~616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO8 to RO7 for the α- and β´-modification, respectively. The Gd2−xEux(MoO4)3 solid solutions are the most efficient emitters in the range of 0<x<1.5, but have a comparable or even significantly lower emission intensity than Sm2−xEux(MoO4)3 for higher Eu3+ concentrations (1.5≤x≤1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the UV-visible-infrared regions of the EELS spectrum.