The water-soluble poly[trimethyl-[7-(3-thienylsulfanyl)heptyl]ammonium iodide-co-thiophene} (PTN+Me3) forms aggregates whose size span a large range of values, depending on solvent, concentration and film formation conditions. Larger aggregates were detected by atomic force microscopy and dynamic light scattering, smaller ones by DOSY NMR, UV-visible and fluorescence. All techniques indicate a reduced aggregation of PTN+Me3 in DMSO relative to water and a marked de-aggregation of PTN+ Me-3 in water following addition of bile-acid anions such as deoxycholate and ursodeoxycholate. The latter effect shows itself through a very large enhancement of the polymer photoluminescence likely caused by disruption of aggregated non-emissive exciton traps. A tailored combination of electrostatic and hydrophobic interactions between the polymer and the bile-acid anions seems to be necessary to achieve this effect. These observations suggest that this polythiophene might provide a basis for the development of fluorescent sensors suitable for the detection of medium-sized amphiphilic biomolecules. (C) 2011 Elsevier Ltd. All rights reserved.