The aluminum monohydroxide pseudoboehmite is a common precursor for catalysts and ad/absorbents. Fibrillar pseudoboehmite powders were synthesized by the same preparation method from aqueous sols but aged in different times: 24, 48 and 168 hours. The sol particles were from fibrillar pseudoboehmite; the average length of the microfibrils changed sample to sample, the sizes increasing with the thermal aging time. The sols were dried by different methods: at room temperatures, forming a solid membrane; spray dried; dried at 60°C and 110°C from a gel from the sol aged 168 hours; all were fired between 200°C and 1000°C in oxidative atmosphere to produce activated transition aluminas. The phases formed was identified by XRD. The aim of the present paper is to present data showing that method of drying the sol has an effect on the temperature of formation of alpha-alumina due to the anisodiametric shape of the pseudoboehmite crystals.