Measurements of electrical conductivity and P-wave velocity of seven rock samples were made in the laboratory under inferred in situ conditions. The samples were collected from smectite and chlorite alteration zones in boreholes from the Krafla and Hengill, Iceland, geothermal areas. The measurements were done in the 25-250 degrees C range, with pore pressure and confining pressure equal to inferred in situ hydrostatic and lithostatic pressures, respectively. Conductivity increases linearly with temperature over the 30-170 degrees C range: that rise is considerably smaller above 170 degrees C. Time-dependent effects on conductivity occur above approximately 100 degrees C. These effects may be related to ion exchange between the clay minerals or the Stern layer, and the pore fluid. The temperature coefficient of conductivity is found to be considerably higher than attributed to pore fluid conduction alone, indicating interface conduction in an electrical double layer on the mineral-water interface in the pores. The results also show that there is no distinction in electrical conduction mechanism in the smectite and chlorite alteration zones: both are dominated by interface conductivity under in situ conditions. The sharp decrease in conductivity at the top of the chlorite alteration zone, commonly observed in resistivity surveys in high-temperature geothermal systems, is most likely due to the lower cation exchange capacity of chlorite compared to that of smectite. (c) 2009 Elsevier Ltd. All rights reserved.