A key parameter affecting the flow of gas in coal is the wetting potential of gas, in comparision to water, to spread over the wall of coal micropores and microfissures. Wettability is quantified in terms of the contact angle of the fluid interface with the solid surface. A fluid with a small angle of contact would spread over the pore walls and eventually displace the non-wetting fluid. Depending on the nature of the coal, gas type and environmental conditions in coal reservoirs, either water or the gas phase could wet coal more strongly. Furthermore, in mixed gas conditions, one gas may be more strongly attached to coal than the other gases. In water-saturated coal, gas desorption in small pores -where most adsorbed gas is stored - can be totally inhibited by water if it is a strong wetting phase. Reducing the hydraulic head (drawdown to achieve the gas desorption pressure) should allow desorption of gas in larger fractures, whereas in small pores, gas desorption could be inhibited by capillary pressure due to the effect of interfacial tension and gas-wetting properties of coal. In this study, we built a new system to quantify the wettability of coal by gas. The contact angle of the water-gas interface with the coal surface inside the gas phase was measured using a captive gas bubble technique. The contact angles of CH4 and CO2 bubbles in water with a coal from the Sydney Basin were measured at different gas-water pressures of up to 15 MPa for CH4 and 6.1 MPa for CO2. The results show that as gas bubbles dissolve in water, the contact angle of the bubble with the coal surface reduces. The contact angle values were smaller for CO2 gas than CH4, and in general, the contact angle value decreases as gas–water pressure increases.