We discuss methods to extract information on the texture from nitrogen adsorption-desorption data of mixed SiO2-ZrO2 aerogels prepared under different conditions. The data have been analyzed following two methods : (a) the classical Brunauer-Emmett-Teller (BET) theory(1) and extensions(2) and the "fractal FHH" theories.(3,4) We show that (a) the classical methods are unable to interprete adsorption - desorption data obtained for this kind of system and (b) fractal FHH theories lead to the conclusion that mixed SiO2-ZrO2 aerogels are surface fractals, independently of the preparations conditions.