A new concept for the mode of action of cationic biocides is proposed in which the antibacterial activity of cationic disinfectants is ascribed essentially to molecular organizations of cations within aggregates, i.e. the activity is determined by the size of aggregates and number of active molecules comprising the aggregate. On the basis of the new concept, the morphological effect of disinfectants in aqueous solution on the antibacterial activity is discussed for low molecular weight phosphonium salts with single and double long alkyl chains (carbon number 14). The proposed new concept can be applied to all phenomena reported previously in antibacterial activity of cationic biocides and this concept is very important from the viewpoint of molecular design of more active cationic biocides.