The maximum liquid fraction that stable pneumatic foam can support is studied in this paper in order to illuminate change of flow regime from turbulent foam flow to the "emulsion regime" in which the distinct interface between the foam layer and the bubbly liquid is lost; this regime transition is nominated as "flooding". With further increases in gas flow rate, large (or "gross") bubbles can appear and rise in the column, the existence of which depends on the absolute pressure at the column bottom. Experiments are carried out using foam stabilized by sodium dodecyl sulfate, and the liquid fraction of both the foam layer and the bubbly liquid are measured using a pressure gradient method. The maximum liquid fraction is found to be at a critical point when the interface between the foam and the bubbly liquid layer disappears. The predicted maximum liquid fraction in the foam is given by epsilon* = (n - 1)/(n + 1), where n is an adjustable constant that depends upon the characteristics of the gas liquid interface, and the predictions have been experimentally verified.