A study was initiated to investigate the effects of gaseous and super-critical carbon dioxide (CO2) adsorption on bituminous coal strength. Uniaxial compressive strength (UCS) experiments were conducted on bituminous coal samples from the southern Sydney Basin saturated with gaseous CO2, super-critical CO2 and N-2 at various pressures, and a temperature 33 degrees C. According to the results, gaseous CO2 adsorption causes the UCS and Young's modulus of the bituminous coal to be reduced by up to 53% and 36%, respectively. Super-critical CO2 adsorption causes more significant modifications to the mechanical properties of the bituminous coal, resulting in 40% greater UCS strength reduction and 100% greater Young's modulus reduction compared to gaseous CO2 adsorption. The greater influence of super-critical CO2 on the UCS of the bituminous coal is thought to be related to the greater adsorptive potential and coal swelling produced for super-critical CO2. The more significant influence of super-critical CO2 on the Young's modulus of the bituminous coal is thought to relate to the greater dissolution (and thus coal plasticization) potential of the super-critical CO2. N-2 saturation was not observed to have any significant effect on the mechanical properties of the bituminous coal. Acoustic emission data collected during testing support of the notion that the coal mass natural cleat system largely contributes to the susceptibility of coal to mechanical weakening by CO2 adsorption. The results show that the mechanical influence of CO2 adsorption on coal is highly dependent on the phase state of the CO2. (C) 2013 Elsevier Ltd. All rights reserved.