In this work we studied the symmetry properties of the Rayleigh-type optical mixing signal, of a two-level molecular system immersed in a thermal bath and irradiated by a classical electromagnetic field. Taking into account that the solvent produces an inhomogeneous displacement of the Bohr frequency and, employing a cumulant expansion to solve the stochastical equations derived, we obtained the mean values of the coherence, populations and susceptibilities Fourier components of the molecular system, from the optical stochastical Bloch equations (OSBE). These symmetry properties are used to show the dependence of the spectra with the frequency shifts of the incident fields, and we obtained that the inclusion of the thermal bath diminishes the intensity response as well it promotes the loss of the symmetry properties, compared with the same results in absence of the bath. © 2013 SPIE.