The influence of surface pressure and adsorption time on the size and shape of J aggregates of 3,3'-disulfopropyl-5,5'-dichlorothiacyanine sodium salt (THIAMS) adsorbed onto dioctadecylammonium bromide (DODAB) monolayer were investigated. After deposition of the monolayers on a hydrophobic glass substrate by the Langmuir-Schaefer (LS) technique, confocal fluorescence microscopy and atomic force microscopy (AFM) were used to study their fluorescence and topography. The difference in size and shape of the J aggregates formed at different surface pressure and adsorption times may be caused by a different rigidity and charge density of the DODAB film. The total thickness of the films amounts to 4.5 +/- 0.2 nm which confirms that THIAMS forms 2D aggregates with edge-on orientation when adsorbed to a DODAB monolayer. In the bulk of the J aggregate the fluorescence was polarized in all films investigated. THIAMS molecules adsorbed to the dark areas between the fluorescence domains remained mainly as monomers and dimers.