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The lake waters of Kawah Ijen volcano are concentrated hyperacidic brines that obtain their composition from volcanic gases and water-rock interaction. Lake water and hydrothermal fluids seep through the western flank of the volcano where they precipitate gypsum and form the hyperacid Banyu Pahit river. In this contribution, we present compositional data from bi-weekly samples taken from the Banyu Pahit between 2015 and 2018 to evaluate whether these fluids can be used to monitor the hydrothermal system, and compare the fluids to the record of variability documented by the precipitated gypsum. Understanding the causal links between variability of the Banyu Pahit and the magmatic-hydrothermal processes operating in the Kawah Ijen system, and establishing a baseline for quiescence are pre-requisites to using the river for monitoring. Fluid samples were analysed at the Ijen observatory for SO 4 /Cl ratio using BaSO 4 turbidity and AgCl gravimetry, and subsequently sent for ICP-MS and ICP-OES analyses. The data shows a first order seasonal variation that can be explained by variable input of neutral groundwater, but is underlain by more subtle trends. Temporally, there is a shift in the system from 2015–2016 to 2017 when the fluid contains a higher proportion of elements derived from rock alteration. The shift occurs at the peak of the rainy season and coincides with a substantial rainfall increase in 2017 compared to 2015–2016. At the start of 2018 there is a marked increase in Cu, coinciding with a gas burst in March 2018. We suggest that an increased meteoric water input led to an increase in water-rock interaction and dissolution of a Cu-sulphide phase in the hydrothermal system. The variability of the fluids over the 3-year sampling period does not exceed that of the longer record preserved by the gypsum precipitates, indicating that it is typical for the current style of activity. • Regular sampling and analysis of hyper-acidic river water at Kawah Ijen between 2015 and 2018 • Changes in fluid sources and sinks were identified using Principle Component Analysis. • Increase in meteoric fluid led to enhanced water-rock interaction and dissolution of Cu-sulphide phase. • The increase in Cu in the fluid coincides with a gas bursting event in 2018. • The variability of the fluids is less than in the longer gypsum-precipitate record. [ABSTRACT FROM AUTHOR]