Abstract: Available interphase area is an important part of investigating catalytic distillation process by the use of a nonequilibrium stage rate model.In this paper the available interphase area,when gas and liquid flow countercurrently in catalyst bed within a distillation column,was measured by using the chemical method in which CO2 in the air is analyzed by Na2CO3-NaHCO3 buffer solution with AsO2 as catalyst.The surface renewal theory by Danckwerts was employed for data processing.