The aim of the paper is to show the potential of the X-ray tomography to quantitatively measure volume changes in granular materials during silo flow in a rectangular model bin. The experiments were carried out with initially dense sand. The bin walls were smooth or very rough. In the first step, continuous X-ray radiographs were conducted. The results of volume changes were presented in form of 10 cross-sectional plots and 20 images. They were directly compared with corresponding experimental results obtained with a Particle Image Velocimetry (PIV) method allowing for measuring displacements on the solid surface. Measurement errors in both methods were discussed. (C) 2010 Elsevier B.V. All rights reserved.