Dy3+ and Pr3+ doped Ga5Ge20Sb10S(Se)65 glasses provide good emission efficiencies in the mid-infrared (mid-IR). The complex glassy network of these chalcogenide glasses were characterized by Raman scattering and extended X-ray absorption fine structure, to put in evidence a structure combining tetrahedra of Germanium and Gallium and trigonal pyramids of Antimony. The arrangement of these structural units permits the introduction of rare earth ions thanks, in particular, to a charge compensation generated by the Ga in a tetrahedral site. Consequently, Pr3+ and Dy3+ ions inserted in such low phonon energy glasses emit efficiently in mid-IR, between 3.5 and 5 μm. Finally, Dy3+ and Pr3+ doped Ga5Ge20Sb10S(Se)65 fibers were obtained from bulk glass preform. Efficient emission in mid-IR was obtained by pumping Dy3+ doped Ga5Ge20Sb10S65 and Pr3+ doped Ga5Ge20Sb10S(Se)65 fibers at 920 nm and 2 μm, respectively.