We have measured isotropic Raman CH stretching spectra of a cyclopentane-d9/CO2 gaseous mixture at various pressures up to 48.2 bar (700 psi). With increasing pressure the spectra broaden and exhibit partial motional narrowing due, in part, to changes in the pseudorotation dynamics. The shape of the experimental spectrum at each pressure can be closely reproduced by a calculation based on a Langevin equation for pseudorotation which incorporates the effect of the of the solvent in a friction parameter. Over the pressure range examined here, the solvent friction is found to scale linearly with the CO2 number density. © 1993.