The absorptive and emissive behavior of four highly fluorescent bisalkyl-substituted poly(p-phenyleneethynylenes) (R = hexyl, dodecyl, 2-(ethyl)hexyl, and 3,5,5-trimethylhexyl) was examined. Aggregate and excimer formation was seen in solution and in the solid state by a combination of absorption and fluorescence spectroscopy in solvent/nonsolvent (chloroform/methanol) mixtures and in thin films. The aggregates were identified by the occurrence of a sharp absorption centered at 440 nm, undetectable in dilute chloroform solution, and a broad emission at 504 nm. The average size of the aggregate formed in chloroform/methanol solvent mixtures must exceed 1.5 mu m, as was evidenced in filtering-experiments utilizing a set of membranes with different pore sizes to successively remove smaller aggregates.