The paper explains the influence of the increase in the volume of coal bed during its thermoplasticity stage on the process of char porous structure formation. Basing onrself on the volume balance of the particular elements of coal bed a mathematical model has been developed for the description of the influence of changes in bed volume as well as its solid phase during carbonization on the total porosity of originating chars. The model has subsequently been used to evaluate the possibility of determining the porosity by means of microscopy, mercury porosimetry and helium method. In order to give a detailed description of the impact of bed expansion on the formation of char porous structure, the pores have been divided into three groups, i.e. pores with radius <7,5 nm, pores with radius 7,5-2500 nm and pores with radius >2500 nm. The division has made on the basis of different influence of factors like carbonization temperature, bulk density and coal grain size on the volume of these pore groups. As a result of the comparison of the pore volume in chars obtained in the conditions of constant volume and those obtained in the conditions of free expansion, the increase in the volume of the bed has been observed to significantly influence only the formation of the pores with radius >2500 nm. Basing oneself on the examination of pore volume in chars produced from single coals and those obtained from their blends, it has been stated that volume of pores with radius <2500 nm is of additive nature. In the case of pores with radius >2500 nm their volume is additive nature only in chars obtained in the conditions of constant volume of the carbonization space. The results presented in this paper are based on laboratory research carried out in the conditions similar to those in commercial coking plants (the rate of heating, the composition and pressure of gaseous atmosphere, the bulk density and the size of coal grains). For this reason the obtained results and conclusions that have been drawn, can be used to explain the process of coke porous structure formation in the conditions of a commercial coking chamber.