The acidity of alginate aerogel films has been investigated by infrared spectroscopy of adsorbed NH3. Supercritical drying of the alginate provided samples with a surface area of several hundred square meters per gram, in which 19 the probe molecule could reach all acidic sites. Free carboxylic groups were studied on acid-gelled alginates and were found to behave as effective Bronsted acid sites. lonotropic alginate gels formed by alkaline earth cations presented only the Lewis acidity of the cations. lonotropic gels formed by transition metal cations presented both Lewis and Bronsted sites, because of the presence of a fraction of free carboxylic groups. The incomplete salification 14 was correlated to the pH of the gelling solutions.