The influence of a sand additive on the transport of the plastic mass during carbonization of two orthocoking coals was studied using X-rays. For this purpose, special markers were introduced into the coal charge. During heating the coal charge with the markers in it, X-ray pictures were taken. Based on the X-ray pictures, the data on the displacement of the charge and change in its volume were obtained at various carbonisation stages. It was stated that the addition of sand has a varying effect on mass transport processes in the coal plastic layer and on the volume of the coal charge. When coal Z is heated, an increase in the sand amount leads to a rise in the charge volume at the stage of coal gains swelling but it does not affect the processes of transport in the gas-saturated zone of the plastic layer. When heating coal P, a 1.8 times increase in sand reduces the distance over which the plastic mass displaces in the gas-saturated zone. The increase in sand in the two coals reduces the shrinkage of the semi-coke. (c) 2005 Elsevier B.V. All rights reserved.