Solute-solvent collisions for a two-level molecular system interacting with a four-wave mixing signal are analyzed in the present contribution. The system is described using the optical stochastic Bloch equations (OSBE), where Bohr's frequency is treated a stochastic variable due to the random collisions with the solvent. The resulting equations for the Fourier components associated to the coherence are averaged over all the realizations of the stochastic variable, using an approximant for the Voigt's function as a probability distribution. In this model we were able to calculate the optical susceptibilities at different frequencies, depicted as numerical surfaces for the behavior of the optical properties. © 2009.