Hydrogen and carbon monoxide mixtures are main reducing agents for gaseous direct reduction (DR) processes. In the present study, volume change of pellet reduced in H-2-CO mixtures at 800-1000 degrees C was examined with a novel consecutive imaging method. Results showed that pellet expansion intensified with temperature and CO content in atmosphere and it reached the maximum at reduction degree of 20%-40% due to the formation of massive wustite phase. Pellet in H-2 rich atmospheres passed the wustite stage rapidly and swelling consequently weakened. Iron whiskers that formed in the later reduction stage led to persistent swelling as pellet reduced by CO. However, pellets contracted obviously in H-2 containing atmospheres for the collection of iron phase in the later. Compressive strength of pellets during reduction dropped to be the lowest in 10-20 min. And the time for pellets remained in the "low strength zone" (<500 N/P) prolonged with temperature and CO proportion in reducing gas, which showed good correlation with their swelling behavior. (C) 2014 Elsevier B.V. All rights reserved.