In analyzing the stability of separated flow (annular or stratified flows), Barnea and Taitel (1989, Chem. Engng Sci. 44, 325-332; 1992, Int. J. Multiphase Flow 18, 821-830) suggested that one should distinguish between the stability of the interface due to the Kelvin-Helmholtz analysis and the stability of the steady-state solutions with respect to the average film thickness, even if the interface is unstable. This type of analysis is termed the structural stability analysis. The stability of the structure was analyzed by a simplified approach (assuming a uniform film thickness). In this work the results obtained by the structural stability analysis are verified by the use of a more exact formulation, the two-fluid model equations.