The coherence of a two-level system immersed in a thermal bath and irradiated with an electromagnetic field was calculated through the optical stochastical Bloch equations. The solvent effects are modeled as a stochastical shift of the system's Bohr frequency and the electromagnetic field includes both pump and probe fields. The methodology includes pump and probe fields to obtain third-order responses. This result only considers a correlation function related to a white noise. The analytical expression of the solution contains the sum of 16 terms, which resonant frequencies are studied theoretically. Finally, the optical properties of a two-level molecular system in presence and absence of the thermal noise, are presented. © 2012 by Nova Science Publishers, Inc. All rights reserved.