We investigate linear and non-linear absorptive and dispersive properties for a homogeneously broadened molecular two-level system including simplified versions of vibrational structure and intramolecular coupling. The Born-Oppenheimer electronic energy curves for this simplified molecular model consist of two intercrossing harmonic oscillators potentials with different force constants. These electronic states are coupled and inclusion of the intramolecular coupling within a vibronic model renders the corresponding coupled states. We have analysed the influence of the intramolecular coupling parameters on the optical response of the system up to third-order in the external field. The critical quantities for this analysis are the transition and permanent dipole moments (both in magnitude and relative sign) which depend strongly on the intramolecular coupling. Some simple exact results are derived for the important case of exact degeneracy. Finally, the relevance of the optical response of the change in force constants associated with the two uncoupled states is examined. © 1994, IOP publishing Ltd.