M o 0 . 5 W 0 . 5 O 3 nanoparticles were prepared through the Pechini process and were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), FT-IR spectrometer, and differential thermal analysis (TG-DSC) analyses. The polyesterification reaction, as the starting step, has a profound influence on the dispersion of the resulting nanoparticles. The molar ratios CA？:？TM？=？2 and EG？:？CA？=？1.5 are favorable for the preparation of M o 0 . 5 W 0 . 5 O 3 nanoparticles having average particles size ranging from 2 to 9？nm. Meanwhile, the molar ratios CA？:？TM？=？4 and EG？:？CA？=？0.19 are favorable for the preparation of M o 0 . 5 W 0 . 5 O 3 nanoparticles having an average particles size ranging from 11 to 29？nm. For the calcination step, increased calcination time (eight hours) at 500°C is advantageous for allowing the monometallic phases enough time to transform into the desired bimetallic M o 0 . 5 W 0 . 5 O 3 phase. 1. Introduction Molybdenum trioxide (MoO3) and tungsten trioxide (WO3) are well-known metal oxides with similar physical and chemical properties. They show n-type semiconducting properties related to the presence of lattice defects, mainly oxygen defects [1, 2], and they have been extensively studied for their potential applicability in gas sensing devices [3, 4] and catalysis [5, 6]. Additionally, due to the unique activity of trioxides of W and Mo in nonstoichiometric forms, they have been extensively studied as electrochemical materials [7]. The MoxWl？xO3 system exhibits “displacive” or “reconstructive” phase transitions induced by hydrogen intercalation and temperature, respectively, leading to rearrangement in the local electronic and atomic structures [8]. Polycrystalline MoxWl？xO3 (x = 0.1, 0.2, 0.3, 0.5, 0.7, and 0.9) solid solutions were obtained by high-temperature synthesis [9]. With the progress in nanotechnology, nanoparticles have attracted increasing attention to their unique properties [10]. One of the recently investigated methods for the simple preparation of nano-oxide composites that include molybdenum or tungsten atoms is the Pechini method [11, 12]. However, research into the catalytic activity of molybdenum-tungsten bimetallic oxides is rare despite their promising gas sensing potential [13, 14]. In this work, the synthesis of molybdenum-tungsten oxide nanoparticles is presented. The procedure includes the formation of composites containing both molybdenum or tungsten atoms in a 1？:？1 ratio using the polymeric method. The influence of citric acid and ethylene glycol concentrations, as well as total
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