[en] Sibayak is located in a young volcanic area of North Sumatra, Indonesia. The surface manifestations such as acid springs, fumaroles and acid alterations indicate that Sibayak geothermal field is probably associated with the volcanic system. The aims of this study are to define the origin of the geothermal fluids, reservoir temperature and interactions between the volcanic system and the geothermal reservoir. Chemical composition, stable isotopes ¹⁸O_H2O, ¹⁸O_SO4, D_H2O, ³⁴S_H2S, and ³⁴S_SO4 of water and gas samples from the geothermal surface manifestations and exploration wells are analyzed. δ¹⁸O and δD values show that the reservoir fluids of Sibayak geothermal field come from meteoric water that is recharged at an elevation of 1300-1500 masl. Geothermometers based on δ¹⁸O_H2O-SO4, water chemistry (T_Na-K-Ca) and gas chemistry (T_H2-Ar) show similar reservoir temperatures that range from 250 to 280 deg. C. δ¹⁸O, δD isotope composition of the steam vents at the cone of Sibayak mountain, having the value of -2.9 per mille for δ¹⁸O and -44.9 per mille for δ D, shows magmatic inputs. (author)