Coal and gas outburst remains one of the most severe dynamic hazards to many underground coal mining operations worldwide, posing great threats to mine safety and productivity. To understand the influence of gas composition on coal and gas outburst propensity, bulk coal samples were collected from underground coal mine sites in NSW and QLD, and subjected to experimental studies. Isotherm adsorption experiments were carried out using the gravimetric isotherm testing method to investigate the impact of coal seam gas composition on gas adsorption characteristics with a range of coal sample particle sizes, to a maximum gas pressure of 4 MPa, at 35°C. The seam gas composition employed in the tests included 100% CH4, a gas mixture of 50% CH4 and 50% CO2, and 100% CO2. For all test coal samples of different particle sizes, the adsorption capacity of CO2 was observed to be the highest, followed by the CO2/CH4 mixture and CH4. For a given gas content, the equilibrium gas pressure of a CH4 rich coal sample is significantly greater than the equivalent CO2 rich coal sample. Given that gas pressure provides energy to induce outbursts, it is reasonable to suggest that CH4 rich coal contains greater outburst initiating energy. Hysteresis occurs during the CH4 and CO2 sorption and is calculated by an improved hysteresis index (IHI) method. CO2 sorption hysteresis is more significant than CH4 sorption hysteresis, with the ratio of IHI_CO2/IHI_CH4 ranging between 1.50 and 2.25. At equivalent adsorption gas content, the amount of CO2 desorption is less than that of CH4, making it difficult to provide sufficient supply of desorption gas, resulting in low gas desorption energy, which is less conducive to the development of outburst. The research results can provide useful theoretical support for mine site gas management in underground coal mines, particularly those operating in areas with moderate to high composition of CO2 seam gas.