Nanocrystalline Gd(2)O(3) products doped with Er(3+) and Yb(3+) cations were synthesized by combustion method. We showed that the temperature of combustion can be tuned by using different types of organic fuels within reaction mixture. The combustion process is performed at lower temperature in the presence of urea as an organic fuel leading to pure cubic Gd(2)O(3):Er,Yb phase, on the other hand higher combustion temperature, yielding pure monoclinic Gd(2)O(3):Er,Yb polymorph is achieved in the mixture of urea and b-alanine. Effective doping of Er(3+) and Yb(3+) cations within Gd(2)O(3) were confirmed by XRD analysis. Both polymorphs show upconversion in green and red areas and possess strong dependence of fluorescence intensity ratios (I(525)/I(549) or I(525)/I(560) for monoclinic and cubic polymorphs, respectively) on temperature. This indicates that both phases can be applied for the temperature sensor devices based on upconversion intensity ratio changes.