Coal mines naturally emit gases, mostly methane, during exploitation and at the beginning of flooding. Once they are flooded, the conditions of gas migration are changed, and the capacity of release and retention of gas by coal is poorly known. The experimental device CASPER has made it possible to demonstrate and quantify a significant release of methane from coal under more than 3 MPa of hydrostatic pressure. The analysis of these results showed that the main fraction of released methane comes from sorption sites. The porous volume has a minor role in reaching content equilibrium, but has a strong effect on hydrostatic pressure, due to water penetration into the coal structure. (c) 2012 Elsevier B.V. All rights reserved.