Alumina with an urchin-like morphology has been synthesized from Al-2(SO4)(3)/CO(NH2)(2)/H2O/isopropanol mixture via hydrothermal route followed by calcination. Raman spectra and SEM images have shown the urchin-like structure to be formed of gamma-AlOOH phase with the component ratio Al3+/CO(NH2)(2)/H2O = 1:(2-3):(100-200). The increase in CO(NH2)(2) content has resulted in the formation of needle-like crystals of NH4Al(OH)(2)CO3. Structural transformations of alumina during hydrothermal synthesis have been investigated by SEM and TEM methods. The 5-10 mu m urchin-like alumina particles have been found to be composed of 100-200-nm-thick sheets. The alumina samples have been studied by low-temperature nitrogen adsorption method. The adsorption-desorption isotherms and pore size distributions have shown that pore structure of the urchin-like alumina is formed by the slit pores with two open ends. It has been found that after treatment with chlorosilanes the surface of the urchin-like alumina becomes superhydrophobic, the average contact and sliding angle values being 154A degrees and 5A degrees, respectively. Addition of 5 wt% of the hydrophobized urchin-like alumina particles to the model fire-extinguishing powder has made the powder hydrophobic, the average contact angle being increased up to 151A degrees. Introduction of 5 wt% of the hydrophobized urchin-like alumina particles to ammonium dihydrophosphate has resulted in a decrease in the cohesion strength for ammonium dihydrophosphate fire-extinguishing powder from 0.415 to 0.104 kPa. The hydrophobized urchin-like alumina particles can be used as functional additives to the fire-extinguishing powders for the improved flowability and water repellency to be achieved.