Metashale cures from The Geysers Curing Project were tested under laboratory-simulated geothermal-reservoir conditions: their electrical resistivity - with and without pore-pressure control, with confining pressures up to 100 bar, and between 20 and 150 C - strongly suggests that boiling occurs gradually. rather than suddenly, as pore pressure is reduced. Gradual boiling is modeled as an effect of capillarity in the fine pores (submicron diameter) of the cores. Model results suggest that boiling in vapor-dominated systems is controlled by temperature and pressure and by pore-size distribution. These experimental results suggest that changes in reservoir conditions can be monitored by surface or borehole electrical-resistivity surveys. (C) 2001 CNR. published by Elsevier Science Ltd. All rights reserved.