The aggregation of the fluorescent hairy rod, anionic conjugated polyelectrolyte poly{1,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) has been studied in aqueous solutions by molecular dynamics simulations, fluorescence and light scattering. Formation of clusters leads to considerable increases in light scattering, decreases in the fluorescence quantum yields and red shifts in emission maxima. Molecular dynamics simulations considering two isolated tetramers in aqueous solution show that they rapidly form aggregates, and support experimental evidence for the association of polymer chains involving both electrostatic and hydrophobic interactions. They also provide indications for proximity of aromatic rings, which is likely to be the main factor responsible for the observed fluorescence behaviour. However, there are no indications of extensive π-stacking. The organic co-solvents methanol, acetonitrile and dioxane break up these aggregates. From studies of the dependence of the aggregation behaviour on dielectric constant or the empirical solvent parameters E^N _T and B_KT for binary mixtures with water, it can be seen that this is not simply an effect of changing solvent polarity, but is due to preferential solvation of the polymer chains. This is supported by molecular dynamic simulations on two tetramers in water–dioxane mixtures (70 : 30%). It is suggested that similar factors are involved in both the association behaviour and aggregate disruption with other hairy rod conjugated polyelectrolytes in water.