The investigation of fluidization of coarse particles displaying the features of B/D and D Geldart＇s groups has been carried out. Hence, the range of the study comprised a transition region, for which only a small number of experimental data are available, whereas the grain-size range of solid particles belonging to these groups is most frequently used in the practical fluidization of coal processing. As a solid material there has been used char of the density of 1500 kg/m(3) and grain size from 0.1 to 3.15 mm (I and II experimental series) and of the density of 1165 kg/m(3) and grain size from 1.0 to 3.0 mm (III series). Three series of experiments with the application of two kinds of gas distributors have been carried out. The kind of the distributor used determined the range of fluidizing air velocity; large velocities have been obtained for the distributor with smaller holes. The investigations comprised the measurement of the fluidized bed height as well as of the height of eruption of gas bubbles. The tests have been performed at increasing and decreasing fluidizing agent＇s velocities. Basing on the literature four models have been chosen, comprising equations leading to the determination of bed expansion, splash zone height and mean porosity of fluidized system comprising fluid bed and splash zone. The obtained calculational results have been compared with the experimental ones. This comparison indicates that there is a big disparity between them (the calculational values being much lower than the experimental ones), which renders the application of the mentioned models impossible. There have been worked out own correlations which enable determining the bed expansion, splash zone height and mean porosity, referring to the material used and the conditions of the experiment. Viz.: [GRAPHICS] The range of application for own correlation is: 0.1 less than or equal to d(p) less than or equal to 3.15 mm, 1165 less than or equal to p(s) less than or equal to 1500 kg/m(3). The analysis of the obtained results confirms the correctness of the elaborated equations. The relative error for the bed expansion in the majority of cases does not exceed +/-10%, in the case of splash zone height - +/-25%. In the case of mean bed porosity the errors do not exceed +/-10%. The results of investigations discussed in this work, referring to the fluidized bed expansion and mean porosity, are significant for the design of the fluidized-bed vessels.