We have observed fifth-order optical response from liquid CS2 by six-wave mixing with five different short pulses and investigated the inhomogeneity in the spectral broadening of the low-frequency intermolecular vibrational modes. There are a couple of distinct differences in the time profile of the fifth-order signal from that of the third-order signal. The fifth- and third-order signals obtained experimentally are compared with the prediction by the theory of Tanimura and Mukamel. We simulate the third- and fifth-order signals in the two cases, the homogeneous limit case and the partially inhomogeneously broadened case with a Gaussian distribution function. The experimental results can be reproduced well in terms of two vibrational modes, one overdamped mode in the homogeneous limit and one underdamped mode with an inhomogeneous width of 2.5 ps(-1).