Fluorescence quenching has been used to study the cooperative conformational transition in a series of oligo(phenylene ethynylene)s having tri(ethylene glycol) monomethyl ether side chains. In nonpolar solvents such as chloroform, the intensity of fluorescence emission from the backbone chromophore increases smoothly as the chain lengthens from the dimer through the octadecamer. In polar solvents such as acetonitrile, on the other hand, chains having more than eight units exhibit fluorescence quenching concomitant with the growth of an intramolecular excimer-like band. This observation is consistent with pi-stacking of aromatic rings for chains that are long enough to fold back on themselves, driven by solvophobic interactions. Titration experiments in which the-solvent composition was gradually changed from pure acetonitrile to pure chloroform showed sigmoidal curves characteristic of a cooperative transition. These data were analyzed using a two-state approximation and a model in which the free energy difference between conformational states depends linearly on solvent composition. The stability of the ordered state was found to increase linearly with chain length, suggestive of a;regularly repeating conformation such as a helix. Therefore the data were fit to a two-state helix-coil equilibrium model where good agreement was observed. The parameters obtained from this analysis revealed a highly cooperative transition driven to fold by strongly interacting monomer pairs.