In this work, we have studied the Four-wave mixing (FWM) absorptive and dispersive optical properties in a two-level molecular system interacting with a thermal reservoir. The nature of the molecular collision induces frequency shifts, which lead to stochastic modulation of the central frequency. This shift is represented in the present model by ξ(t)=ω0+σ(t), where σ(t) comprises all the stochasticity in the function ξ. Modifications in the Optical Conventional Bloch Equations are induced by this shift, and the solution in the Fourier space is given by using the convolution theorem. The topological behavior of the nonlinear optical properties is analyzed as function of the chemical concentration of the absorbent species and in terms of the line shape distribution. The propagation effects of the all electromagnetic fields are included in this model. © 2006 Elsevier B.V. All rights reserved.