Aggregation in some different tetraalkylammonium dodecanoate systems has been studied. The influence of counterions on the phase behaviour of both binary surfactant/water systems and more complicated, multi-component systems were investigated, by applying NMR self-diffusion and Langmuir Blodgett techniques. Increasing the size of the tetra_x001F__x001F_alkylammonium counterions was found to lead to a substantial reduction of the critical micelle concentration. This is due to increased ion-micelle interactions associated with the enhanced hydrophobicity of the larger counterions. Furthermore, the increase in the ion size was found to be correlated with a depression of the tendency of the surfactant to form aggregates with low surface curvature, such as liquid-crystalline phases and rod-like micelles. The importance of the counterion size with regard to the formation of microemulsions in surfactant/water/oil systems was also demonstrated.