Cellulose aerogels with variable specific surface areas were prepared by freeze-drying cellulose microfibrillar suspensions dispersed in mixtures of water and tert-butyl alcohol (TBA). Varying the composition of solvent used for the suspensions and the origin and composition of the microfibrils allowed to extensively tune the specific surface area of these aerogels, with values ranging from 32 to 280 m2/g. Depending on the solvent mixture composition, these aerogels presented substantial morphological differences, attributed to the templating power of the various crystals obtained during the freezing of the solvent mixtures. The aerogels were hydrophobized with sebacoyl chloride in a gas-phase reaction operated under conditions of either full or partial derivatization of the accessible surface. For fully grafted specimens, a linear correlation could be established between the overall degree of substitution of the grafted samples and the specific surface area of the aerogels before grafting. From the slope of this correlation line, an average number of grafted molecules per surface area of approximately 3 molecules/nm² could be calculated. Atomic-scale models gave a plausible overview of the full surface coverage of the aerogel microfibrils in good agreement with the experimental data. Contact angle measurements confirmed the hydrophobicity of the grafted aerogels, as well as the presence of heterogeneities for the partially grafted samples.