High-temperature X-ray powder diffractometry was performed on pure and impregnated (with sulfate and phosphate additives) Zr(OH)(4) while being in situ heated (in air) to different temperatures up to 1000 degreesC. Accordingly, portions of these materials were ex situ calcined at some selected temperatures within the temperature regime examined. The calcination products were subjected to Fourier-transform laser Raman and infrared spectroscopic analyses. The results showed the pure hydroxide to decompose into low-temperature stable monoclinic zirconia (m-ZrO2) in a slow process intermediated by metastable tetragonal (t-Zr-O-2) and/or cubic (c-ZrO2) phases. The metastability of the intermediate phases was, therefore, considered to be a consequence of the slow kinetics of conversion. Moreover, high surface energy (small particle size) of these phases was suggested as an additional reason for their existence, in terms of consistent literature data. The sulfate and, particularly, phosphate additives were shown to slow further the formation of m-ZrO2. This has been suggested to arise from (i) suppression of particle sintering (growth), and (ii) phosphate stabilization (most likely) of c-ZrO2. Results of the present investigation may help a better understanding of phase transitions occurring during the thermal genesis of zirconia-based solid acids from an amorphous parent material, a domain that has hitherto received little agreement in the literature.