The residence time distributions (RTD) of the gas stream elements have been investigated in a column packed with bi-dispersed system composed of static bed of uniform spheres and of sands filling at rest uniformly the voids between the spheres or forming organised fluidised phase generated by the gas stream. The improved input-output dynamic method has been employed. Spheres and sands fractions were mixed in various combinations in individual series of experiments, and air was used at flow rates varied from that slightly below the sands fluidisation onset to that initiating sands entrainment. It has been found that the RTD is mono-modal when the air flow rates are below those at the sands fluidisation onset, or when they promote the homogeneous fluidised phase. In the range of the air flow rates exceeding slightly that at the fluidisation onset, the RTD becomes multi-modal, and this indicates the existence of several flow zones in the air stream generating the fluidised phase. For the bidyspersed system, at the flow rates below that at the fluidisation onset, the modal time can be predicted and the values of the dispersion coefficient are smaller than those in the mono-dispersed system. In the range of air flow rates promoting the homogeneous fluidised phase in the bidyspersed system, the modal times are shorter and the dispersion coefficients are larger than the corresponding values observed in the mono-dispersed bed of uniform spheres at the same flow rates. As the flow rates are increased, and consequently - as the porosity of the fluidised phase is increased, the values-of the modal time and of the dispersion coefficient approach those characteristic of the mono-dispersed bed of uniform spheres.