The cationic, all-conjugated AB diblock copolymer poly[9,9-bis(2-ethylhexyl)fluorene]-b-poly[3-(6-trimethylammoniumhexyl) thiophene] bromide (PF2/6-b-P3TMAHT) shows dual fluorescence from the poly(fluorene) (PF) and poly(thiophene) (PT) blocks. A comparison of fluorescence quenching of the cationic PT block fluorescence with unquenched PF block provides a sensitive ratiometric method for anion sensing. The application to analysis of halide ions, single- and double-stranded DNA is demonstrated. High selectivity is observed with halide ions, with the strongest quenching being seen with iodide. The quenching with DNA can be used for nucleic acid quantification at sub-M concentrations.