A novel water-soluble poly(para-phenylene) derivative with pendant thymine and sulfonate units (PBTS) has been prepared and its metal ion sensing properties have been investigated. PBTS exhibited a reversible and selective fluorescence quenching behavior toward Hg2+ ions as compared to Ag+, Ni2+, Mg2+, Ca2+, Hg2+, Co2+, Cd2+, Cu2+, Pb2+, Ba2+, Fe3+, and Zn2+ ions in aqueous solution. The fluorescence quenching resulted from the interpolymeric pi-stacking aggregation which was induced by the specific thymine-Hg-thymine interaction.