Hydrogen and carbon monoxide mixtures are reducing agents for main direct reduction (DR) processes. However, sticking of iron ore pellets during reduction always leads to discontinuous operation. In order to evaluate the effects of the temperature, syngas constituents and reduction degree on sticking properties, reduction of iron ore pellets in the presence of H-2-CO mixtures was conducted at 800-1000 degrees C The experimental results indicated that fibrous iron on the sticking interface hooked the pellets together at lower temperatures, while sintering of fresh iron with high activity led to the rise of sticking index (SI) at high temperature. SI decreased with the addition of H-2 in reducing gas for the porous iron precipitation on the interface. Sticking firstly occurred on edge angles and then developed connection in large area as reduction degree increased. Bonding of wustite phase could be attributed to the sticking at lower reduction degree. Additionally, crystallization of iron phase and formation of low melting eutectic phase became the dominant factor as reduction degree increased. (C) 2012 Published by Elsevier B.V.