The interstage backmixing in a multistage, mechanically-agitated, compartmented (MSAC) column was determined experimentally, using air-water. The effects of impeller speed, impeller diameter and both superficial liquid velocity and superficial gas velocity through the interstage opening were investigated. Both the counter-current and co-current flow of air and water in the column were investigated. As air superficial velocity through the opening increased from zero, the interstage backmixing significantly decreased initially, reached a minimum, then increased slowly to a maximum after which it decreased continually with further increase in superficial gas velocity. The effect of the gas flow on interstage backmixing was correlated in dimensionless form.