The water-soluble fully sulfonated polyaniline, poly(2-methoxyaniline-5-sulfonic acid) has been found to photoluminesce in its emeraldine oxidation state, ES(PMAS). Excitation of aqueous solutions at 450-530 nm, near its characteristic absorption band at 473 nm, led to emission at ca. 600 nm (2.1 eV). Calculations incorporating cyclic voltammetric data for ground state PMAS suggest that the photo-generated ES*(PMAS) excited state should be both a very powerful oxidising agent and a strong reducing agent (ca. 2.7 and 1.1 V of oxidising and reducing power, respectively). This prediction has been confirmed from photolysis studies of ES(PMAS) in the presence of the potentially reducible cobalt(III) cations CoCl(NH3)(5)(2+), Co(H2O)(NH3)(5)(3+) and Co(en)(3)(3+). In water, photo-oxidation of the ES(PMAS) to its fully oxidised pernigraniline base PB(PMAS) form was confirmed by UV visible spectroscopy. In the case of Lambda-Co(en)(3)(3+) as the acceptor, strong support for the concomitant reduction of the Co(III) complex to the corresponding Co(II) species was obtained from its observed racemisation. Relatively low quantum efficiencies (<3%) were established for these novel, photo-initiated redox reactions. These photo-redox reactions were preceded by a photo-initiated conformational change in the ES(PMAS) polymer from an "extended coil" to a "compact coil" conformation. The presence of added Na+ and NH4+ ions had a marked influence on the nature of the photo-reactions, with Na+ ions retarding the photo-assisted electron transfer, while NH4+ ions prevented the initial conformational change in the ES(PMAS). (C) 2003 Elsevier B.V. All rights reserved.