Prior to development, the Matsukawa geothermal field was partially vapor-dominated. The youngest mineral assemblage consists of early pyrophyllite, diaspore and pyrite, and later anhydrite and quartz, implying deposition from an acidic, high-temperature fluid. Fluid inclusions in anhydrite and quartz from core and cutting samples collected in wells drilled in the western upflow zone of the field were studied to characterize the temperatures and compositions of these late fluids. The results of fluid inclusion studies indicate that the temperatures during the deposition of anhydrite and quartz were up to several tens of degrees lower than the reservoir temperatures at the time of exploitation. Fluids trapped in anhydrite had temperatures of Lip to 257 degrees C, CO2 concentrations in the 0.4-2.6 mol% range and salinities of 1.9-11.3 wt.% NaCl. This compositional variation is related to vapor loss occurring during boiling. The data suggest that the geothermal reservoir is currently being reheated by subvolcanic intrusions. (c) 2006 CNR. Published by Elsevier Ltd. All rights reserved.