Abstract: The invention relates to a process for preparing water-soluble &pgr;-conjugated polymers by polymerizing, in aqueous solution using an oxidant, monomeric thiophene derivatives of formula (I) wherein X and Y are identical or different and are O, S, or N—R′, Z is —(CH2)m—CR2R3—(CH2)n—, R1 is aryl, C1-C18-alkyl, or hydrogen, R2 is hydrogen or —CH2)s—O—(CH2)p—SO3−M+, R3 is —(CH2)s—O—(CH2)p—SO3−M+, M+ is a cation, m and n are identical or different and are an integer from 0 to 3, s is an integer from 0 to 10, and p is an integer from 1 to 1.