The underground circulation of hot water, of interest for geothermal energy production, is often indirectly inferred from the presence of minerals formed by hydrothermal alteration at different temperatures. Clay minerals, such as smectite and chlorite, can be mapped from the surface using electrical soundings and give information about the structure of the geothermal system. Here, we investigate the specific role of smectite in the electrical response of igneous basaltic rocks and evaluate what physical processes make smectite a better electrical conductor than surrounding minerals. Laboratory measurements of cation exchange capacity (CEC), mineralogy, porosity and electrical conductivity are presented for 88 core samples from four boreholes at the Krafla volcano, Northeast Iceland. CEC is found to be a reliable measure of the smectite weight fraction in these volcanic samples, through a comparison with an independent quantification of the smectite content using Rietveld refinements of X-ray diffraction patterns. The bulk electrical conductivity, measured at fluid conductivities in the range 0.02-11.7 S m −1 , increases non-linearly with the fluid conductivity for samples with high smectite content. This non-linear variation is fitted with a function and a model for a conduction process through connected interlayer spaces within smectite. The process differs from electrical double layer conduction, which involves only cations on the crystal edges of smectite, not in the interlayer spaces. The laboratory results can help refine interpretations of electrical soundings in the context of geothermal exploration.