The long-term performance of the reservoir is essential in order to ensure competitive life-cycle cost of the geothermal installations. Geothermal fluids are often saturated with gasses such as CO2 and N2. With their extraction from the reservoir, pressure and temperature decrease towards the extraction well. This disturbs the state of equilibrium the geothermal water is in with its dissolved components, which for gas can lead to exsolution. The exsolved gas bubbles can block the pores of the reservoir rock and therefore reduce the apparent permeability. As permeability reduction occurs mainly near the extraction well it can reduce production of geothermal waters substantially. This paper is aimed at experimentally investigating the conditions at which the onset of degassing starts and quantitively assess any associated permeability decrease. Knowledge on these parameters will enable operators to adapt their operation procedures in order to ensure long-time reservoir permeability. This paper reports core-flood experiments where tap water containing dissolved carbon dioxide was injected into either a Bentheimer (2.3 Darcy) or Berea (140millidarcy) sandstone core at different conditions. The first sets of core-flood experiments showed that at a temperature of 30 °C and pressure up to 50 bars the onset of the degassing process correlates closely to CO2solubility values obtained by the Henry’s law. At these conditions CO2 degassing near the core outlet will cause the apparent permeability to decrease by a factor2 to 5 in the high permeability Bentheimer sandstone core. At the same conditions the apparent permeability will decrease by a factor of nearly 10 in the low permeability Berea sandstone core. The decrease ineffective permeability is gradual in the Bentheimer sandstone while in the Berea sandstone the change is steeper. For rocks with small pore sizes and low absolute permeability, the reduction in effective permeability is larger and the rate of permeability decrease is faster. However, the onset of degassing is not influenced by the pore size and initial permeability. Experiments at temperatures between 30 and 90 °C show that with increasing temperature, the Van ‘t Hoff equation becomes less to accurate to find the degassing pressure