Sol-gel silica coatings doped with an acid indicator were prepared and their behaviour vs pH in aqueous media were studied. Absorption spectroscopy was used to characterize the reversible optical response of doped films both in liquid and gaseous media. Several features such as: a) strong influence of acid concentration on films colour change, b) reversibility without fatigue all over the pH range, and c) spontaneous regain of initial basic absorption when coatings were sensitized under acid vapour; pointed out that dye molecules were incorporated in the surface of the silica films pores. On the other hand, coatings resistance vs alkaline attack showed that damage quickly proceeded if films surface were inhomogeneous, due to direct corrosion of the glass substrate.