Accurate testing of coal isotherms plays a significant role in the areas of coal seam gas drainage, outburst control, CO2 geo–sequestration, coalbed methane (CBM) and enhanced coalbed methane recovery (ECBM). Coal isotherms were tested with CO2 and CH4 gases with a modified indirect gravimetric method in the University of Wollongong (UOW), Australia. The adsorption capacity of coal shows a linear decrease with increasing temperature. The decreasing rate of adsorption capacity is greater for CO2 than CH4, and which is also higher for dry coals. The moist coal sample isotherms for both CO2 and CH4 demonstrate a much lower sorption capacity. The average reduction ratio of coal adsorption capacity of CO2 and CH4 with moisture is 11.1% and 14.0%, respectively in the testing temperature range. The experimental results show that moisture content in coal has greater effect on the adsorption capacity of CH4 than that of CO2.