We have investigated the influence of halogen counterions on the molecular organization characteristics of dioctadecyldimethylammonium bromide (DOMA) monolayers at the air/water interface. Surface pressure- and surface potential-area isotherms and Brewster angle microscopy were used to characterize the head group-counterion interactions. The counterions used were F-, Cl-, Br- and I-. The presence of halogen ions in the subphase leads to compression of the pi-A and a decrease in the surface potential. The effects of counterions on the microscopic features of molecular organization are, however, the most dramatic. DOMA at the air/water interface shows a homogeneous and featureless morphology during the whole compression process. The presence of F-and Cl- ions in the subphase does not lead to any significant changes. However, the introduction of Br- ions results in the appearance of dendritic structures in the liquid/solid coexistence region. The I- ions have an even greater effect on the morphology, in that solid domains are formed immediately after spreading. The influence of counterions on DOMA isotherms and morphology may be rationalized in terms of the degree of hydration of the various ions.