In this study, the bubble point pressure effect or, as it is more commonly known, the flashing point effect of CO2 on geothermal fluids is shown, and the compressibility of the geothermal fluids containing low concentrations of dissolved CO2 at the flashing point is formulated for isoenthalpic phase change. The compressibility, termed the isoenthalpic flashing point compressibility, can be calculated with well-known parameters. New, easy-to-use graphs are presented to estimate the compressibility for such systems. Correction for ionic strength of the geothermal brine is also considered. The practical use of the compressibility is illustrated to estimate the fluid content of a geothermal reservoir, A material balance method incorporating the isoenthalpic flashing point compressibility in geothermal modeling is investigated. The material balance method presented in the paper is of primary importance in the evaluation of geothermal reservoirs since both the areal extent and the vertical thickness can not be accurately determined, particularly in the initial development stages of the reservoirs, Applications of the Method to geothermal fields are discussed. (C) 2002 CNR. Published by Elsevier Science Ltd. All rights reserved.