Water-soluble poly(3-thiophene acetic acid) (P3TAA) and its copolymers with 3-n-methylthiophene or with 3-n-octadecylthiophene were synthesized. The solution properties of these water-soluble polythiophene carboxylic acids were studied by potentiometric titration, viscosity measurements, and UV-visible spectroscopy. We have found that the Henderson-Hasselbach plot of the titration curve showed an inflection of the slope around pH = 5-6. The reduced viscosity and electronic spectra of the P3TAA aqueous solution also showed an abrupt change at this pH region. These results are quite different from that of a common flexible polyelectrolyte, such as poly(acrylic acid) (PAA) and strongly suggest that the pH-induced abrupt conformational changes occurred from the aggregated state to the ex-tended state of the polymer main chain. The observed results are explained in terms of the rigid hydrophobic main chain and the nature of the conjugated polymer.