Mixed titanium (IV) and zirconium (IV) alkoxides were co-hydrolyzed either at room temperature or at 140 °C in the presence of ammonium-containing catalysts such as hydroxide, carbonate, chloride or nitrate. Mesoporousaggregates of Zr-doped (5 mol%) titania have been obtained at 140 °C and in the presence of catalyzing NH4OH or NH4Cl, respectively, whereas only microporous products resulted in performing the remaining treatments. By calcination at 400 °C the microporosity of the products drastically was reduced while their mesoporosity was preserved, contemporaneously the crystallization of anatase-type TiO2 was favoured. The formation mechanism of mesopores in the absence of a templating agent is a consequence of a thermal interconnection of the primaryparticles with packed inter-particle spaces responsible of the mesoporosity. Such formation is discussed in terms of nature of catalyzing agent, alcoholic media, temperature and treatment time of hydrolysis.