Gd3+and Sm3+co-activation, the effect of cation substitutions and the creation of cation vacancies in the scheelite-type framework are investigated as factors influencing luminescence properties. AgxGd((2−x)/3)−0.3−ySmyEu3+0.3☐(1−2x)/3WO4(x= 0.50, 0.286, 0.20;y= 0.01, 0.02, 0.03, 0.3) scheelite-type phases (AxGSyE) have been synthesized by a solid-state method. A powder X-ray diffraction study of AxGSyE (x= 0.286, 0.2;y= 0.01, 0.02, 0.03) shows that the crystal structures have an incommensurately modulated character similar to other cation-deficient scheelite-related phases. Luminescence properties have been evaluated under near-ultraviolet (n–UV) light. The photoluminescence excitation spectra of AxGSyE demonstrate the strongest absorption at 395 nm, which matches well with commercially available UV-emitting GaN-based LED chips. Gd3+and Sm3+co-activation leads to a notable decreasing intensity of the charge transfer band in comparison with Gd3+single-doped phases. The main absorption is the7F0→5L6transition of Eu3+at 395 nm and the6H5/2→4F7/2transition of Sm3+at 405 nm. The photoluminescence emission spectra of all the samples indicate intense red emission due to the5D0→7F2transition of Eu3+. The intensity of the5D0→7F2emission increases from ~2 times (x= 0.2,y= 0.01 andx= 0.286,y= 0.02) to ~4 times (x= 0.5,y= 0.01) in the Gd3+and Sm3+co-doped samples. The integral emission intensity of Ag0.20Gd0.29Sm0.01Eu0.30WO4in the red visible spectral range (the5D0→7F2transition) is higher by ~20% than that of the commercially used red phosphor of Gd2O2S:Eu3+. A thermal quenching study of the luminescence of the Eu3+emission reveals the influence of the structure of compounds and the Sm3+concentration on the temperature dependence and behavior of the synthesized crystals. Ag0.286Gd0.252Sm0.02Eu0.30WO4and Ag0.20Gd0.29Sm0.01Eu0.30WO4, with the incommensurately modulated (3 + 1)D monoclinic structure, are very attractive as near-UV converting phosphors applied as red-emitting phosphors for LEDs.