The broadband multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy in the frequency domain was used to study six-wave mixing processes of simple aromatic molecules such as benzene and toluene. The six-wave mixing CARS signals were observed to have similar spectral profiles to those of the four-wave mixing CARS signals, but the peak positions were located in the overtone and the combination regions. By examining the concentration dependence, the six-wave mixing CARS signals were attributed to the so-called parallel-cascading third-order process. It is shown that the parallel-cascading process is useful for differentiating the contribution of the inhomogeneous broadening to the spectral profile. (C) 2003 American Institute of Physics.