The Cidanau geothermal field is situated in the northern part of Mt. Parakasak in Banten province, West Java, Indonesia. Nine hot spring samples, one river sample, and one cold spring sample were collected and analyzed for their chemical composition and stable isotope ratios of the water. The objectives of this study are to characterize the chemical composition of hot spring water, to estimate the reservoir temperature and to develop a model of a hydrothermal system using chemical composition and water isotope ratios. The geothermal waters were characterized mainly as Na-Cl-HCO3 type of water. The pH of the waters was close to neutral. The temperature of the hot-water discharges varied from 32.2 degrees C to 61.1 degrees C. The waters were evaluated to be immature due to mixing with shallow groundwater during the upflow process. Mixing ratios between cold and hot waters were estimated to be 12.2 to 98.3% with Cl concentrations using a mass balance approach. Higher mixing ratios were found for samples at KR-5, KR-37 and KR-43 and can be explained by the existence of fault zones. The delta O-18 and delta D values for geothermal waters in the region ranged between -7.1 parts per thousand to -6.4 parts per thousand, and -43 parts per thousand to - 38 parts per thousand, respectively. Isotope signatures of all hot springs suggest a local meteoric origin, and the oxygen shift corresponds to an exchange between circulating meteoric water and reservoir rocks in the geothermal system. A conceptual model of the Cidanau geothermal field hot springs is proposed.