Experiments were performed and semi-empirical correlations were developed to both prove the existence of gas-phase secondary flow and to predict annular to stratified transition limits for isothermal and heated horizontal annular two-phase Bows in pipes. At the low vapor velocities, where this transition from stratified flow occurs and the entrainment/deposition mechanism is insignificant, secondary Bow in the vapor core plays the principal role in the full development of the liquid annulus. Direct measurements of secondary Bow are presented along with a simple model to correlate its behavior. The secondary flow model is used in conjunction with the fluid mechanics of the film to derive a boundary model for the onset of annular flow in this transition region. The resulting boundary agrees well with both the thermal and isothermal transition data.