Scintillators with efficient conversion of high-energy radiation into luminescence are needed for application in the new experiments in fundamental physics as well as in the medical imaging, new security control systems, etc. Though RE-doped crystals are in the main trend nowadays, some applications (e.g., cryogenic scintillating bolometers) require novel materials with the intensive intrinsic luminescence. Actually crystals with activator’s emission become inefficient at low temperatures due to the self-trapping of charge carriers by the host. Compounds with NbO4 complex (niobates) are characterized by the bright intrinsic luminescence in a wide temperature region, whereas the highest density can be achieved in compounds with TaO4 complex (tantalates). It can be expected that RE(TaxNb1-x)O4 mixed crystals will combine the advantageous properties of both compounds. Another feature of mixed crystals is non-linear dependence of light output on the concentration of substituted ions [1,2]. However the processes, which determine the observed nonlinear dependence, are still under discussion. Here we present the study of two sets of mixed tantalate – niobates with different cations (Y and Gd), which demonstrate different dependence of light output on x value and therefore allows to control the light output by cation substitution. The features of energy transfer and origin of luminescence mechanism are discussed taking into account the band structure of the crystals.     Samples of RE(TaxNb1-x)O4, where RE = Y or Gd, and x=0, 0.2, 0.4, 0.6, 0.8, 1, have been synthesized by solid-state reaction. The luminescence characteristics of the samples were measured using synchrotron radiation at the branch-line FINEST at MAX-lab, Lund and using various laboratory set-ups under UV, VUV, electron and X-ray excitation. Luminescence spectra, excitation spectra, decay curves and thermostimulated luminescence (TSL) curves were measured in the temperature region 10 – 400 K. Calculations of the band structure were performed using planewave based first-principle methods.   The intrinsic emission observed in Y(TaxNb1-x)O4 is peaking at 2.8 eV and connected with excitons, self-trapped at NbO4 (x ≠ 1) or TaO4 (x = 1) groups. The corresponding emission is quenched in Gd(TaxNb1-x)O4 and was observed using time-resolved luminescence technique only. The emission band related to the defects of crystal structure is observed for both set of mixed crystals and this emission dominates in Gd(TaxNb1-x)O4. The dependence of light output on x demonstrate different behavior – increase of intensity for intermediate values of x in Y(TaxNb1-x)O4 and linear dependence in Gd(TaxNb1-x)O4 (T = 300 K). The origin of the observed difference is discussed. The modification of the bandgap with x value has been studied as well. The gradual shift of TSL peaks to lower temperatures with increase of x indicates the decrease of bandgap. This supposition is verified by the band structure calculations and data of luminescence excitation spectra.