Abstract: CO gas is often used as an effective indicator gas for the prediction and early warning of coal spontaneous combustion, but the causes for the rapid decrease or even disappearance of CO after the closing of spontaneous combustion in goaf are not clear, which affects the accurate determination of the coal spontaneous combustion degree. In order to study the adsorption/desorption characteristics of CO gas in coal, the pore structure of coal samples was determined by pressured-mercury testing and liquid N₂ adsorption experiment. The effects of coal samples with different particle sizes on CO gas adsorption/desorption characteristics were investigated at 303.15-333.15 K and 0.15-0.50 MPa by using a self-developed gas adsorption/desorption device, and the adsorption rate and desorption hysteresis effect of CO gas were analyzed in depth. The results showed that the pore volume of non-cohesive coal sample from Lingxin Mine was mainly large pores and transition pores, accounting for 33.02% and 38.26%, respectively. The pore specific surface area was mainly micropores and transition pores, accounting for 97.73%. With the diminishment of particle size, the proportion of micropore volume increased, and the proportion of transition pore and mesopore decreased. The adsorption amount of CO gas on coal samples with varing particle sizes increased with the rising of pressure. When the pressure was constant, the adsorption amount of CO gas decreased with the rising of temperature. Under the identical temperature and pressure conditions, the smaller the sample particle size was, the larger the CO gas adsorption amount was; at the same temperature, the saturated adsorption capacity of coal for CO was positively correlated with particle size. In the process of CO desorption, the saturated adsorption capacity increased with decreasing particle size. The CO adsorption rate of coal samples could be divided into three stages: rapid rising period (0-750 s), slow rising period (750-2 250 s), and saturated equilibrium period (2 250-3 600 s). The CO gas desorption hysteresis with varing sample particle sizes decreased with the rising of temperature. At the identical temperature, the smaller the sample particle size was, the smaller the desorption hysteresis of CO gas was, and the easier it was to reach the equilibrium state of adsorption/desorption.