The negatively charged adduct formed by reaction of 2,4-dinitroaniline with NaBH4 (Meisenheimer complex) has been successfully incorporated inside faujasites. The internal location of the anionic Meisenheimer guest was confirmed by XPS. This anionic reaction intermediate becomes largely stabilized by the inclusion in zeolite as the result of an increase of the association constant with alkali metal ions inside the zeolite cavities. (C) 2004 Elsevier B.V. All rights reserved.