Aqueous solutions of N,N'-bis(2-phosphonoethyl)-3,4,9,10-perylenediimide (PPDI) were titrated with sodium dithionite, generating radical anions (PPDI-center dot) and dianions (PPDI2-). In aqueous ethanol (1:1 v/v), PPDI existed as monomers and remained in the monomeric form in all reduced states. In water, on the other hand, PPDI formed dimers, which were transformed into a pi-stacked aggregated form upon one-electron reduction to PPDI-center dot. Addition of a second electron, however, resulted in dissociation of the aggregates, giving the dianion PPDI2- in the monomeric form. The presence of CTAB micelles prevented the aggregation of PPDI-center dot in water, indicating that the radical anions were incorporated into the micellar phase. Reduction of PPDI in ethanol solution, where the dye was aggregated, resulted in the formation of soluble mixed stacks containing both PPDI-center dot and PPDI2-. These mixed stacks stabilized the radical anion form, preventing further reduction to the dianion.