A new non-invasive, non-destructive experimental technique for process investigation is introduced. In it a steady stream of gas is passed through the system. At a suitable moment, the composition of the gas is changed and this causes a composition front to move through the system. The chemical system is chosen so that a temporary change in the interaction between the flowing gas and the system as the front advances causes a change in the effluent flow rate. Measuring the change in the effluent flow rate reveals an effect of the internal structure of the system. When the pneumatic impedance of the system is small, changes in effluent flow rate occur almost simultaneously with the front passing regions where there is spatial variation of the interaction between the flowing gas and the system. It is sometimes advantageous to change the flow rate in addition to the gas composition. It is possible that composition-front scanning could augment and enhance tomographic techniques. The method of composition-front scanning was demonstrated by experiments in which the interaction is physical adsorption. It was found to be possible to deduce the transit time of the front across a small inactive zone in a model adsorptive porous medium from the effluent flow-rate change during a scan. This experimentally determined transit time agrees with that calculated from the size of the inactive zone and the flow rate, In another case the presence of a small adsorptive zone in an otherwise inactive porous medium was demonstrated.