The self-association of the antidepressant drugs amitriptyline, nortriptyline, doxepin and desipramine hydrochlorides in aqueous solution has been studied by static and dynamic light scattering at 298.15 K in the presence of added electrolyte (0.0–0.4 mol kg⁻¹ NaCl). Self-assembly was by a closed association process, commencing at a well-defined critical concentration for all systems. Micelle properties determined from static and dynamic light scattering techniques as a function of electrolyte concentration were related to differences in the structure of the hydrophobe. The pair interaction potential between aggregates has been discussed in terms of the DLVO theory of colloidal stability.