An optical probe for chromate ion has been obtained by the sol-gel technique by depositing a porous film containing the reagent diphenylcarbazide on a glass support. Changes in the absorption of light as a result of the (irreversible) formation of a purple product between chromate and the reagent inside the sol-gel are monitored. The detection limit for chromate anion is as low as 1 ppb, but only after rather long exposure to samples. The morphology of the surface of the film has been studied using atomic force microscopy and is related to the technique of preparing the film.