Aqueous solutions of N,N'-bis(2-phosphonoethyl)-3,4,9,10-perylenediimide (PPDI) were titrated with sodium dithionite, generating radical anions (PPDI(-*)) and dianions (PPDI(2-)). 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(-*). Addition of a second electron, however, resulted in dissociation of the aggregates, giving the dianion PPDI(2-) in the monomeric form. The presence of CTAB micelles prevented the aggregation of PPDI(-*) 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(-*) and PPDI(2-). These mixed stacks stabilized the radical anion form, preventing further reduction to the dianion.