Cyclization rate measurements were carried out on polystyrene chains of different molecular weights and labelled at both ends with I-pyrenyl groups, using as solvents cyclopentane (a thetasolvent), acetone (a poor solvent), and their mixtures. These mixtures exhibit a cosolvency effect for polystyrene. The results show that the rate constants for cyclization and ring-opening are solvent-and chain-length-dependent. This was interpreted in terms of the influence of excluded volume effects on these processes. The results indicate that the solvent giving the largest excluded volume effect is an equimolar mixture of cyclopentane and acetone, which agrees with the light scattering and viscometry results for polystyrene in cyclohexane/acetone mixtures.