Doping behavior of two kinds of water-soluble polythiophenes (PThs) (PTh with a CH2CH2CH2SO3M substituent at the 3-position, P3(RSO3H)Th, and poly(ethylenedioxythiophene), PEDOTh) has been investigated spectroscopically at various pH under air. The p-doped (or oxidized) state of P3(RSO3M)Th is stable at pH = 1.0, whereas at pH levels higher than 4, the polymer is undoped. To the contrary, the p-doped state of PEDOTh is stable over a pH range of 1 to 7. Mixing of PEDOTh and a DNA (poly(G-C)(2)) in a Tris buffer solution leads to a change of the UV-visible spectrum and precipitation of their adduct, supporting formation of an adduct between PEDOTh and poly(G-C)(2). (C) 2000 Elsevier Science B.V. All rights reserved.