Thermostability of GdMoO4Cl within temperature range 25-1500 degrees C was studied by DTA, X-Ray powder diffraction, chemical and optical methods. Compound is relatively stable up to 470 degrees C, it exhibits polymorphic transformation at 1030 degrees C. This suggests the decomposition scheme to polyphase products: 4GdMoO(4)Cl O-2 --> -2Cl(2) Gd-2(MoO4)(3) + Gd2MoO6. Compounds of composition LnWO(4)Cl and Ln(3)WO(6)Cl(3) identified by X-Ray powder diffraction and some physical and chemical methods were reported to form solid phase reaction [1-3]. The compounds can be applied in laser technology as efficient phosphorous. We reported optimal conditions for synthesis [4], luminescence, spectral and optical properties [5-7].