Mixing in bubbling gas-fluidised beds comes primarily from bubble motion through the particulate phase and this has previously been studied experimentally by examining the displacement of a marked layer of particles by rising bubbles. The reported experimental profiles show considerable variation in the distortion of the interface. In this work, the distortion of the interface by a two-dimensional bubble of initial radius a, rising inviscidly in a semi-infinite channel of width b, is examined theoretically and the results applied to the interpretation of experimental data from fluidised beds, assuming that the particulate phase may be modelled as an inviscid fluid. Two new contributions are considered : (i) the contribution from the injection of the bubble, and (ii) the effect of initial finite separation, z(0), of the bubble and marked layer. Numerical calculations and analytic expressions show that the distortion of the interface depends on z(0)/b and not z(0)/a. When z(0)/b < 0.4, the distortion of the interface is shown to be critically dependent on the initial separation of bubble and layer. The area between the final and initial position of the layer, the partial drift area (Eames et al., 1994, J. Fluid Mech. 275, 201-223), is equal to the area of the bubble’s primary wake. A review of experimental results shows that many of the results were taken for z(0)/b < 0.4, even though z(0)/a was large. Qualitative comparisons are made between theory and experiment which show some agreement.