The influence of common cationic surfactants on the physical properties of differently composed polyelectrolyte films prepared by the layer-by-layer (LbL) technology was investigated. Free-standing polyelectrolyte films as microcapsules showed a fast, strong response to the addition of less than 1 mM cationic surfactant cetyltrimethylammonium bromide (CeTAB). As a function of the polyelectrolyte composition, the behavior of the capsules varied from negligible changes to complete disintegration via strong swelling. The response of microcapsules consisting of (poly(allylamine hydrochloride)(PAH)/poly(styrene sulfonate)(PSS))(4) was associated with a 5-fold volume increase, a fast switch of permeability, and in the case of fluorescently labeled films a 4-fold increase in fluorescence intensity. The kinetics and strengths of the interaction process were investigated by confocal laser scanning microscopy (CLSM) and fluorescence spectroscopy. Also, the relative stabilities of the polycation/polyanion and surfactant/polyanion complexes were determined. A mechanism was suggested to explain the interactions between the cationic surfactants and polyelectrolyte capsules. The strong response can be exploited in potential applications such as the triggered release of drugs or other encapsulated materials, the fluorescence-based detection of cationic detergents, and a switchable stopper in microchannels. However, the high sensitivity of LbL films to traces of cationic surfactants can also limit their applicability to the encapsulation. of drugs or other materials because pharmaceutical or technical formulations often contain cationic surfactants as preservatives such as benzalkonium salts (BAC). It was demonstrated that undesired capsule opening can be effectively prevented by cross-linking the polyelectrolyte multilayers.