In ladle refining operations, a plume eye is the area of the bath surface that is not covered with slag, due to bubbling gas, and this area is the most important in slag–metal reactions. In contrast, in vacuum degassers, most of the surface area is not covered with slag, and in this case, the bubble rising area at the bath surface is important for gas–metal reactions. This area is referred to as the “bubble eye area” in this paper. To evaluate the bubble eye size, and its effect on the surface reaction rate, water model experiments were carried out. The plume eye size was measured by changing the thickness of a polystyrene particle layer, which was added to the surface as a slag layer. The bubble eye size was estimated by extrapolating the plume eye size to the value calculated for a polystyrene layer thickness of zero. The volumetric mass transfer coefficient of the surface reaction was measured at various size of plume eye. The volumetric mass transfer coefficient at the bubble eye area was evaluated by extrapolating the mass transfer coefficient at the plume eye area to the value calculated for a plume eye size equal to the bubble eye size. It was concluded that the size of the bubble eye is influenced by the gas flow rate through each nozzle, whereas the volumetric mass transfer coefficient at the bubble eye area is influenced by the bubble eye area and the bubble rising velocity.