Electrical resistivity is extensively used in geothermal systems to accurately determine the existing conditions of the reservoirs at depth. Up to this point, technical challenges related to pore fluid confinement made difficult to measure electrical conductivity at temperatures and pressures representative of very deep geothermal reservoirs. In this study, we are overcoming these limitations thanks to a new electrical resistivity cell designed to fit into a high temperature gas medium apparatus. This allows us to perform resistivity measurement at temperatures up to 700 °C and at effective pressures up to 100 MPa (i.e. a confining pressure of 130 MPa and an equilibrium pore pressure of 30 MPa) using cm-scale plugs. Rock samples originate from five boreholes located in the Icelandic geothermal fields of Reykjanes (RN-17B/Hyaloclastite, RN-19/RN-30/dolerites) and Hengill (NJ-17/basalt and NJ-17B/Hyaloclastite). These samples were selected for their high degree of hydrothermal alteration in the epidote and amphibole facies (i.e. temperature of 250 °C and 400 °C respectively), and their wide range of porosities (from 3% to 20%). To determine the effects of surface, mineral and electrolytic conductions on bulk electrical conduction, experiments were performed under dry and saturated conditions using three different fluid salinities.At temperatures ranging from 25 to ~350 °C, electrical conductivity in all our experiments increases as a result of both increasing surface and electrolytic conduction. Then, under supercritical conditions, i.e. temperature from 374 °C to 600 °C, electrical conductivity strongly decreases due to the evolution of water density and dielectric constant that affect both surface and electrolyte conduction. At higher temperatures (500 °C–700 °C), the rock conductivities lie within the range of dry rock electrical conductivity values, suggesting that mineral conduction controls the bulk conductivity with ferromagnesian minerals acting as principal contributors of mineral conduction. Amphibole-rich samples show an irreversible increase in conductivity at temperature above 500 °C–600 °C, which can be attributed to amphibole dehydration. Comparison of these laboratory data to magnetotelluric soundings and downhole temperatures obtained beneath several geothermal areas indicate a good agreement between laboratory and large-scale surveys. Our results provide a general trend that helps interpreting electrical conductivity surveys in the Icelandic crust.