The effective vibrational relaxation time of CO2excited to the 00°1 level has been measured by the phase fluctuation Optical heterodyne technique. An apparent pressure dependence of the relaxation time can be attributed to the finite velocity of sound. This effect can be corrected to yield a value ofptau = 7.6 mus . atm. The effect of laser-initiated cylindrical sound waves on the data is examined in some detail.