We report on the tail-tail cyclization and unfolding kinetics of poly(dimethylsiloxane) that is end-labeled with pyrene (Py-PDMS-Py) when it is dissolved at low concentration in liquid toluene (a good solvent) as a function of added CO2 (0-204 bar). The pyrene excimer emission provides information on the PDMS tail-tail cyclization and unfolding kinetics and the conformation of the polymer chains. Under the aforementioned conditions, the Py-PDMS-Py excimer emission is entirely intramolecular in nature; there is no evidence for any inter-or intramolecular ground-state preassociation of the pyrene residues. However, the pyrene excimer-to-monomer intensity ratio (E/M) increases by similar to 5-fold as we increase the CO2 pressure from 0 to 70 bar. E/M begins to decrease gradually as the CO2 pressure is increased above 70 bar. Time resolved fluorescence spectroscopy reveals three important points. First, the rate of Py-PDMS-Py tail-tail unfolding (k(unfolding)) is essentially independent of added CO2. Second, the rate that describes the intramolecular Py-PDMS-Py tail-tail cyclization (k(cyclization)) increases 5-6-fold between 0 and similar to 90 bar CO2. Above similar to 90 bar CO2, kcyclization decreases with increasing CO2 pressure. Finally, the apparent excited-state equilibrium constant (K* = k(cyclization)/k(unfolding)) increases with added CO2 up to similar to 90 bar and then decreases above 90 bar. The independence of k(unfolding) on adding CO2 suggests that this rate coefficient reports on a local process that is not influenced to any significant extent by chain conformation or the viscosity of the medium. The large change in K* argues that the addition of CO2 affects the PDMS chain cyclization probability which is a manifestation of changes in chain conformation brought on by the addition of CO2. Together these results show that the addition of CO2 to liquid toluene (up to similar to 90 bar) results in a systematic decrease in the mean free distance between the pyrene-labeled PDMS termini. This change in the mean free tail-tail distance is consistent with an excluded volume argument which is in line with Monte Carlo simulations and small-angle neutron scattering experiments. Above similar to 90 bar CO2 the mean free distance between the pyrene-labeled termini begins to increase. This arises from an increase in the solvent quality with increasing CO2 density.