The adsorption of carbon dioxide (CO2) into the coal matrix during CO2-enhanced methane recovery causes significant alterations to the coal mass chemical and physical structures, causing modifications to coal's mechanical properties. Hence, the main objective of this study is to investigate the effects of sequestrated CO2 phase condition on coal strength. A series of unconfined compressive strength tests was conducted on Australian brown coal samples, saturated under various CO2 pressures (2-10 MPa) at 35 degrees C using an advanced acoustic emission (AE) system and optical 3D deformation analysis. According to the results, super-critical CO2 has the ability to cause a greater reduction of strength (by 46%) and enhancement of elasticity properties (by 20%) in brown coal compared to sub-critical CO2, because super-critical CO2 has greater adsorptive potential, which eventually creates greater coal matrix swelling. According to the AE and deformation analysis, the coal mass natural cleat system contributes significantly to the observed CO2 adsorption-induced changes in mechanical properties. (C) 2015 Elsevier B.V. All rights reserved.