The structure and properties of the blends poly[sodium 2-(3’-thienyl)ethanesulfonate] (P3TESNa)/poly(vinyl alcohol) (PVA) and poly[2-(3’-thienyl)ethanesulfonic acid] (P3TESH)/PVA, both with a mole ratio 1/1 of the two components, were investigated by gel permeation chromatography (GPC), X-ray diffraction (XRD), infrared spectroscopy (IR), ultraviolet-visible-near-infrared spectroscopy (UV-vis-near-IR), X-ray photoelectron spectroscopy (XPS), dynamic mechanical analysis (DMA), and conductivity. The blends were cast from water solutions of the two components. The P3TESH/PVA blend consists of the two phases, a P3TESH phase and a P3TESH/PVA complex phase, in which no characteristic of the PVA component is observed. In the complex phase, the two polymers are intimately mixed. However, this compatibility is different from that of conventional polymer blends in that the present blend has an additional phase composed of one of the two pure components. The phase with the complexes has glass transition and side chain relaxation temperatures higher than that-of the pure components, and has new chain packing as reflected in a new X-ray diffraction peak (2 theta) at 21.5 degrees. This type of compatibility results from the hydrogen-bonding (or electrostatic) interaction between the two components and the strong. aggregation of one component (ill this case, P3TESH). The blending of P3TESH with PVA leads to a significant undoping, and the conductivity decreases from 10(-2) S/cm in the pure state to 10(-6) S/cm. The P3TESH subchains that are not self-acid-doped (or containing the -SO3H group) exhibit a red shift of the W-vis absorption maximum by 44 nm, resulting from an increase in coplanarity caused by a repulsion between neighboring -SO3- groups. For the P3TESNa/PVA blend, the phase structure and electrostatic interaction are similar to those in the above blend. A drastic red shift of the W-vis absorption maximum by 76 nm (0.52 eV) accompanied by a color change from pale orange-yellow to bright orange-red after the blending is observed. The red shift results from the strong electrostatic interaction between the Naf ion of P3TESNa and the O atom of PVA. Such a large red shift is equivalent to the thermochromism and solvatochromism of poly(3-alkylthiophene)s. The conductivity of P3TESNa (10(-7) S/cm) drops by 1 order of magnitude after blending with PVA, but the blend can be doped by aprotonic acid to give a conductivity of 10(-3) S/cm.