In 1996, the level of electricity generation in Tongonan geothermal field increased from 112.5 MWe to around 462 MWe, or about 410% increase since 1983. This massive exploitation level has resulted in a rapid decline in field pressure by as much as 4.0 MPa from 1996 to 2001. The present response of the production wells to this pressure decline is a shift from discharges dominated by liquid to discharges dominated by steam, indicating the lateral expansion of the steam zone. The eventualities of this field process are the increase in field enthalpy from around 1600 kJ/kg in 1996 to more than 2000 kJ/kg in 2001, and the corresponding increase in the available steam of production wells from 1050 to 1380 kg/s. The expansion of the upper steam zone in Tongonan left a remarkable geochemical signature of a field with a receding water level. This includes an abrupt drop in the mineralization of the fluids, as shown by the sharp decline in reservoir chloride and silica, simultaneous with an increase in the well fluid enthalpy until the liquid phase of the discharge completely disappeared. In the present condition of the Tongonan field, vapour chemistry and gas equilibria have become highly important monitoring tools of changes across the field. The application of the FT-HSH2 gas equilibria identified a progressive vapor gain across the field, except for the wells affected by the inflow of injected and cooler fluids. The SNHC gas equilibria indicated that the deep source fluids sustaining Tongonan field are possibly dominantly liquid-phase. However, the decline in pressure allowed boiling and phase separation to occur at a deeper level, thus generating a thick and expansive upper steam zone across the field. The SNHC equilibria also identified a depressurized area, in terms of decline in partial pressure of CO2, at the center of the field coinciding with a depressurized area based on well measurements. The trend of CO2 partial pressure may, therefore, become a useful tool for monitoring further pressure changes in the future. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.