A physical model is suggested to explain the occurrence of a shell of varying porosity around a gaseous bubble rising in a fluidized bed. According to the model, this phenomenon is a direct consequence of the necessity to maintain the balance of forces acting on solid particles in the vicinity of the bubble. Both volume forces due to gravity, drag and inertia and surface forces caused by the particulate pressure are shown to be essential. The agreement between model predictions and experimental data is good apart from some difference in the particle concentration at the edge of the bubble void. Requirements of the mass, momentum and energy conservation at this edge regarded as a surface of discontinuity result in conclusions concerning the particle flux through the bubble surface and the solid concentration inside the bubble.