Some cationic amphiphiles with an ammonium head group and a long alkyl chain (n = 18) were found to form homogeneous ion-complexed monolayers with anionic pyrene-1-carboxylate at air-water interfaces. After hydrochloric acid gas treatment of Langmuir-Blodgett films of the monolayers, many fluorescent fine spots with intense emission were observed with a fluorescence microscope. The spots were assigned to microcrystals of pyrene-1-carboxylic acid that was formed in contact with HCl and migrated in the film to form the crystallites.