The metastable hexagonal form of tungsten trioxide (h-WO3) has recently been attracting much interest [1]. The synthesis routes of h-WO3 involve the dehydration of tungstic oxide hydrates, WO3 · nH2O (n= 0.3 [2, 3]; n= 0.9 [4]) or the oxidation of ammonium tungsten bronze [5–7], or ion-exchanging templating ions from hexagonal bronze [8]. Regarding the role and effect of possible residual chemical impurities, considerable uncertainty has remained. In this laboratory, we have prepared the oxide–hydrate precursor, WO3 · 1/3H2O by the route Na2WO4 · 2H2O→ H2WO4 ·H2O → WO3 · 1/3H2O [2, 9, 10]. According to a previous report, the formation of WO3 · 1/3H2O requires the presence of sodium in the reaction system, resulting in sodium incorporation in the WO3 · 1/3H2O phase [11]. This work focuses on the kinetic influence of sodium on the transformation/dehydration of WO3 · 1/3H2O into h-WO3 and on the hydrogen reduction of h-WO3 into elemental tungsten. The dehydration experiments of WO3 · 1/3H2O (of varying sodium contents) into h-WO3 were carried out either at isothermal conditions (300 ◦C, 15, 30 and 90 min, N2–O2 (80–20%) atmosphere) or in thermal gravimetry equipment (Mettler, Registrierender Vakuum-Thermoanalyzer, H2 (5N) ambient) where oxygen loss from the oxide during H2 reduction was also followed. X-ray powder patterns were recorded (at room temperature in a Guinier focusing camera using CuKα radiation, λ = 0.154051 nm) after each isothermal heat treatment and thermal analysis. The positions of the reflection lines on the film were determined by computer-controlled densitometric analysis [12]. Lattice parameters were obtained from the least-squares refinement of the d values of 23–26 reflections in the range 13◦ < 22 < 30◦. The calculated cell parameters for h-WO3 (JCPDS card 33-1387) samples obtained by dehydration of WO3 · 1/3H2O with [Na]= 163– 3420 ppm are collected in Table I. Morphology and grain size of h-WO3 and elemental tungsten were studied by scanning electron microscopy (SEM; Jeol 25). Transparent samples for transmission electron microscopy (TEM; Philips CM 20, operating at 200 kV) were prepared by depositing dispersed crystallites onto a holder, or by ion milling [14, 15]. TEM investigations reveal streaks in the diffraction pattern of h-WO3 (Fig. 1) showing the presence of stacking faults in the lattice. Second phases, precipitates or preferred sites for the allocation of sodium (Na+, Na2O) other than in the hexagonal channel (as suggested in [16]) have not been found; all samples of varying sodium contents, however, have been examined. Sodium concentration of the WO3 · 1/3H2O samples was found to influence the time of heat treatment needed for the dehydration/phase transformation as is demonstrated on XRD spectra and thermogravimetric (TG) plots. Details of the XRD spectra of samples isothermally heated for varying times show the “shift” of the 0 0 4 reflection of the orthorhombic WO3 · 1/3H2O to the position of the 0 0 2 reflection peak of h-WO3 (Fig. 2). This shift, indicating the dehydration of samples, is observed to be faster for the sample with 3400 ppm of sodium than that of samples with 160 and 1050 ppm. In the XRD patterns of the intermediate samples, temporary new peaks also appeared, which cannot be identified either among WO3 · 1/3H2O or h-WO3 peaks. TG curves of the dehydration of WO3 · 1/3H2O samples are plotted in Fig. 3a. In accordance with the observations at isothermal treatment shown in Fig. 2, the influence of the sodium content in the solid phase was also observed on the rate of dehydration. The WO3 · 1/3H2O sample with 160 ppm of sodium dehydrates with the minimal velocity. The dehydration velocity of the samples with 1000 and 3400 ppm of sodium increases with increasing sodium content. Samples with intermediate levels of sodium, 495 and 740 ppm, however, exhibit a greater dehydration velocity than samples with