The aggregation of conjugated polymers in common organic solvents is investigated using fluorescence correlation spectroscopy (FCS), burst analysis, and microscopy. Poly(3-hexylthiophene) and poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylenevinylene] are both shown to form weakly bonded non-emissive aggregates in toluene that persist even at picomolar concentrations. These aggregates decrease the bulk emission :intensity in solution but do riot affect the fluorescence spectra or lifetimes, consistent with a static; quenching mechanism. Passing the solutions through a syringe filter causes an increase in the number of emitters as measured by FCS, indicating that this process dissociates the aggregates. Films cast from solutions that have been filtered are more uniform and significantly more emissive than, those made from unfiltered solutions. These results Show that FCS is a highly sensitive probe of non-emissive aggregates in solution that have a deleterious effect on the emission properties and overall quality of spin-cast thin films, even at sub-nanomolar concentrations.