The acidity of alginate aerogel films has been investigated by infrared spectroscopy of adsorbed NH
3. Supercriticaldrying of the alginate provided samples with a surface area of several hundred square meters per gram, in whichthe probe molecule could reach all acidic sites. Free carboxylic groups were studied on acid-gelled alginates andwere found to behave as effective Br
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nsted acid sites. Ionotropic alginate gels formed by alkaline earth cationspresented only the Lewis acidity of the cations. Ionotropic gels formed by transition metal cations presented bothLewis and Br
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nsted sites, because of the presence of a fraction of free carboxylic groups. The incomplete salificationwas correlated to the pH of the gelling solutions.