The association characteristics of the phenothiazine drug trimeprazine tartrate in water and aqueous electrolyte solution have been examined using static and dynamic light scattering techniques and compared with those reported for the structurally related drug promethazine hydrochloride, to identify differences in association pattern arising from differences in the counterions of the two drugs. The association of trimeprazine tartrate was micellar in water and the the presence of added electrolyte. A progressive increase of aggregation number and micellar radius was observed with increase of sodium chloride concentration over the range 0.05 to 0.60 M NaCl. This behavior is in contrast to that of promethazine hydrochloride which exhibits continuous association above the critical concentration in electrolyte of concentrations greater than or equal to 0.2 M NaCl. Moreover, static light scattering curves for trimeprazine tartrate showed a single discontinuity corresponding to the onset of micellization, whereas the association behavior of promethazine hydrochloride (and other phenothiazine drugs with chloride counterions), in water and dilute electrolyte (0.05-0.1 M NaCl), is complex, with discontinuities in solution properties at several critical concentrations. The results of this study highlight the influence of the counterion on the association characteristics of this class of amphiphile.