The Tiwi geothermal field in Southern Luzon, Philippines, has been in commercial operation since 1979, reaching an installed capacity of 330 MW by 1981. The geochemistry of the produced fluids has evolved through a number of reservoir processes that have been induced by production. These include the influx of cool meteoric waters and injected fluids, the migration of acidic fluids from the southwestern margin of the reservoir, and the development of an extensive steam cap with associated production of superheated steam. Ancillary challenges resulting from these processes include scaling and corrosion of wellbore casings and pipelines. The most serious problem was the massive influx of cool meteoric recharge in the early 1980s, which drowned production in the initial development area. This recharge, however, now provides beneficial mass and pressure support to the current production area. Despite these operational challenges and after more than 23 years of commercial operations, the Tiwi field remains a major geothermal resource. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.