Koch in Ref. 1 presented a procedure for estimating the maximum pressure rise potential of axial flow compressor stages. When used in conjunction with a stage stacking off-design performance prediction method, it can indicate which stage is likely to be responsible for stalling a multistage compressor. Based on Koch’s approach, it is believed that the casing over the stage that stalls first (critical stage) should be treated in order to prevent or reduce the possibility of stalling. With the help of a two-stage compressor test rig, experimental investigations on the stage stalling and the ideal axial location of the treated casing in the multistage environment were performed using a recessed circumferential slot casing treatment. Experimental results indicate that it is possible to predict the stage that stalls first using Koch’s engineering method. In addition, it was verified that if the casing treatment was located over the critical stage, the stall margin of the whole multistage compressor could be improved significantly.